I want to create a character icon like in Dota 2 or StarCraft 2
If you want to make 2D art assets yourself, you’ll need to install some paint software. I suggest Gimp, which is a free alternative to photoshop. If you don’t have a drawing tablet, you can hand draw the lines of your character and then scan it or take a picture with your phone. Then color in the art with Gimp or Photoshop.
You can find thousands of tutorials on drawing, painting and coloring on YouTube.
If you have a budget, you could commission artwork from artists who have a style you like. Deviant Art is a good place to look for inexpensive commissions, but remember that you get what you pay for.
Keep in mind that those games are AAA titles produced by huge companies who have access to top level talent. You aren’t going to reach that quality level without a lot of work or a good bit of money.