The default tool is the pencil, so just click a color you want. You can erase by clicking the erase button in the tools panel on the left of the screen More Tools will be coming after this project gets 50 like, both likes and the star.
Go Follow me on PixilArt too: https://www.pixilart.com/redhatszx 50% Of credits go to PixilArt because I just made a scratch version of their websitle or tool. Other 49% Go to me, and 1% goes to the person who first came up of the idea of making a pixel art software in scratch. It tool me about 2:30 hours to complete so the code is pretty simple. You can remix, but give credit, and change something in the remix you made. This is the scratch rules, the change something in the remix you made part. If you see two of the same colors in the colors picker, you didn't see it okay. ;)