So first, download some brushes from DeviantArt if you haven't yet, and just download them to your computer. Open the folder that you saved them in, and there should be something that says "extract all". Click that, and follow through the instructions.
Then open GIMP, and go File>Preferences. Then go to "Brushes", and press the button that looks like a piece of paper. Then a button with three dots should show up. So click that, and select the folder where you save the brushes, then click OK.
After that, restart GIMP, and you're done!
If that was too hard to follow, let me know. I'm always here to help with GIMP, so feel free to ask me more questions. XD
