To Get Brushes To Work (For Windows):
Okay, first things first, you need to go online and download the latest version of GIMP. I have 2.6 I think, but I'm not sure if there's a new one after that. When you're downloading it, it'll ask you where to put the folder for GIMP files, just for the sake of convenience, put it in the My Documents thing, or on the Desktop so you can access it easier.
Now that you've done that, what you can do.. is go to qbrushes.com (they have REALLY good brushes) and find a brush that you like, and download it. When you're downloading that, put it in the GIMP folder wherever you placed it. After that's finished, it gets a biit harder.
So, you go to where the file is that you just downloaded, it should be right there in the GIMP folder, and you double-click on it to open it. Something should pop up and say something about it being a zipped file, and it should give you some instruction about where the "unzip file" button is, depending on what kind of Windows you have. When you unzip it, there should either be one file and a folder, or a bunch of individual files that have picture of the brushes that you just downloaded. You're going to copy those, and then go back to the gimp folder. You'll see a folder labled "Brushes" which is where you put those individual files.
Now you can use those brushes in GIMP!