I've made this little spaceman sprite, (original size 16x16 just resized it for uploading)
I have problems with making the walking animation, it comes out horrible.
Can somebody help me to make the walking animation fluid and nice?

EDIT: I forgot to say that this is for a platform game

How many frames are you making the walking animation and why is this guy facing toward the viewer if he's a platform runner/jumper??

your question has not much explanation but if you want to make your animation more fluid you can just add more frames to it. you should know one important thing about looping animations like running and walking and... . your animation should end at a certain frame that statrted frame. you can find it yoursef why. and if you dont do that your animation looks laggy

Here's a reference of how we do animations for sprites in our game (they are 32x64). It includes some skinless base models for male, female, and child sprites that we draw features over to make our sprites and animations. Obviously they are a lot different in size from your sprites, but maybe you can use this as a reference for your own animations? Sorry I couldn't be of more help.