well, what tools and hardware do you have at your disposal?
The pros would use something like Photoshop with a wacom tablet to color their linework... Altough I can really recommend Clip Studio Paint / Manga Studio for coloring.
If you have no money to spend, Gimp is a good opensource alternative to Photoshop. Of course, its quite a complex tool, so prepare for some learning before you can get really efficient at using the tool.
If you have no Wacom or similar Pen Tablet, you can also just use the good old mouse for that. Artist were working with the plain old mouse for decades, and if you master it it is almost as effective as a pen tablet.
Of course, you could get really old school and color it the old analog way, print out your design, color it with physical paint, and scan it.
Ehr, about the textures.... go light on the textures for now. Trust me, there is nothing worse than a beginners lineart (don't get me wrong, looks good for someone who just started, but still) coupled with to intense textures tiled into the lineart.
Textures will NOT make your design look cooler. It will look out of place and cheap. That is why you seldom see the pros using textures in 2D pieces (besides with b&w work like manga comics, where textures are a replacement for shading sometimes).
First master the art of clean color fills. Believe me, even with flat colors your design will immidiatly look better.
Next step would be shading. but this will take some time to master.
Be prepared to spend YEARS drawing before you get to a pro-like level. And don't waste too much time looking for shortcuts (like the textures)... there are none, really.
Some might seem to make it easier for beginners, but without training will still not look that great.
1000's of hour of practice to master an art, is how the saying goes....
