Though I really like the process of creating games, I can't think of any interesting idea for a game to make. I made 2 very small games based on my own idea before, but they turns out pretty bad. The only game I made that my friends would even consider playing are tinkered clones of retro games.

Recently while learning phaser I read a tutorial blog article which aslo brefly mentioned how the idea came to the author. It seems surprisingly smooth to him - turning a phrase into an idea, then to a concept. Seems a lot like designing flowery interface for your own blog. I tried to do the same but, nope, nothing promising came after 2 hours.

So how do you normally come up with idea? Is there a brainstorming process or it's always just 'spark of the moment'?

I think it's a combination of both.

When I'm short of new ideas I try to set aside 30 minutes to an hour a day where I just sit down and write down a bunch of ideas on paper. Just list them out as they come to mind. At the end of the week I go through my ideas and pick out the ones I find interesting and throw the rest away. I'll do a little research on the interesting ones and see if they lead anywhere. If not then I'll throw those away as well and start on a new list.

I'll also try to carry around a small notebook in case I'm at the bus stop and an idea hits me. If I don't write it down there's a chance I'll forget about it.

I usually come up with game ideas when I am playing a game and something in it annoys me or seems under-used, under-developed. This is one of the common ways people get inspired to create something (especially fanfiction) but there are other, quite-different methods that work just as well. The method of starting with a phrase is "prompt-based brainstorming" which is another very common method. Some people put words in a bucket and draw random ones, or use an online generator. For novels, I've personally sat in a bookstore or library, written down titles that caught my imagination, then made up a plot to go with my impression of the title. (Usually completely different from the actual contents of the book.) Mind-webbing is a third method, it takes the approach of taking a few key words that are deeply important to you and exploring how they might be related to each other in a fictional or game-design structure.

1. Main character
2. Secondary characters and locations
3. Story context
4. Game genre
5. Reinterpretation of the game genre trough the main story

I can't think of any interesting idea for a game to make.

Why do you need an idea? This is not a frivolous question. Are you looking for something to practice on, so you can up your game dev skills? Or are you looking to build an indie business for yourself, so you need a moneymaking product? Or what?
Most professional designers are given an assignment, a basic concept to design from (so most pro designers don't always need to "think of interesting ideas" out of nowhere per se).

Lots to consider here.

I think the first thing to realize is that design of any kind is not a fire-and-forget process. It doesn't matter what you're designing, a straight line between inspiration and tangible, universally-loved product simply does not exist. The path is twisty, branching, has many dead ends, and often loops back on itself. Those who believe or seek to prove true that there is a straight line are fools. Not only are these 'time wasters' unavoidable, they are a necessary part of the process. Design is not simply having an idea and making it real, design is taking the seed of an idea, and then exhaustively exploring all the space around it to find what works and what doesn't. You develop ways and ideas that compliment each other, and prune away those that don't, even when they might stand perfectly well on their own. Design is the process of finding form, not dictating it.

The skill to design, then, is not in simply having good ideas, but in being able to recognize ideas that enhance and compliment each other by combining to be something that is greater than the sum of its parts. Megaman isn't fun because he's a robot who moves left to right shooting things and gathering new weapons from bosses -- Megaman is fun because of how he moves through a world filled with a variety of enemies that represent different challenges, and which through their behavior, placement, timing, and environment create a novel, compound challenge on nearly every screen, and how by defeating bosses in a different order allows you, the player, to change those challenge equations.

Finally, its perfectly OK to be derivative of or to refine other games or combinations of other game mechanics, as long as you aren't duplicating them or using IP that's not yours. Romeo and Juliette is a story based on a previous tale, and which has been retold a thousand times since. Its a pattern or Genre of story that speaks well to us, just as there are genres and patterns in movies and games. But these things have to be moving, interesting, or fun to be true to what they are -- take it away and they are no longer the thing they claim to be -- a soulless story is not one worth hearing, a dull movie is not one worth viewing, a boring game is not one worth playing. In the entire space of what could be done, there are infinitely more failures to be found than successes, so if you make those core tenants subservient to your design -- rather than the other way around -- its no wonder that you will find yourself in the weeds with your boring game.

This last notion is not unlike the set of all numbers that exist -- in that set of numbers there exists an infinite amount of irrational numbers, unexpressible and forever repeating, and yet we thing of them as uncommon and not normal. In truth, its the rational numbers, those that have an end and therefore we can express, that we cling to in all we do, despite their cosmically singular rarity. Finding a new design that works is tantamount to discovering a new rational number, and its only natural that it might bare some resemblance to others we already know, or from which we might have learned from to find it.

There are many ways to find ideas for games:

Playing other games does help.

Here's one way I've managed to consistently deliver original concepts (not all of them good, mind you):

Pick a piece of code you've done before (say, a pong or pac-man clone).

Look at the good (how it is laid out, the way things have been structured).

Figure out a way to make something totally different out of it in as little time as possible.

Instead on relying on your ability to design, it let's your mind wander around about the possibilities of systems that you've already created (and how flexible they are).

You'd be surprised what you could end up with when converting a pong or pac-man clone into a totally different game (retaining the assets, and only the logic you really need).

The code base generally becomes a mess, but it generally leads to much more than just a 'twist on' the game.

I can't think of any interesting idea for a game to make.

Why do you need an idea? This is not a frivolous question. Are you looking for something to practice on, so you can up your game dev skills? Or are you looking to build an indie business for yourself, so you need a moneymaking product? Or what?
Most professional designers are given an assignment, a basic concept to design from (so most pro designers don't always need to "think of interesting ideas" out of nowhere per se).

Yes I want to try making something new to improve my dev skill. Making clone game within the day might be fun but I don't think I want to spent some weeks or months making a clone game :(

Instead on relying on your ability to design, it let's your mind wander around about the possibilities of systems that you've already created

Said another way: Embrace limitations as a creative opportunity.

Many amateur designers think that a design that has more stuff or a design that eliminates limitations is necessarily better than those that don't. Limitations, practical or otherwise, are the enemy. But a design that attempts to sidestep all limitations is one which loses all focus. I daresay that if the goal was ever achieved, you couldn't even say it was 'designed' at all, impressive as the result may be.

Besides that, complete freedom to do anything is paralyzing. There's so much that you could do that its hard to be clear about what you should do. Having limits, technical or self-imposed, keeps things near, where they are easier to reason about together, and sometimes by that virtue allows for creative solutions to present themselves, without being passed over for fear of unforeseen circumstances.

Design abhors unnecessary complexity. Limitations naturally curtail it.

I want to try making something new to improve my dev skill. Making clone game within the day might be fun but I don't think I want to spent some weeks or months making a clone

If you can't come up with something original, don't. Your goal now is to improve - so make a clone (but one that won't take forever). Do something simple and different from what you did before. Trying to force originality is a side quest down a road that doesn't lead you to your stated goal.

complete freedom to do anything is paralyzing.

Nicely put!