Step 1. First of all what you want to do is acquire some brushes. How can you install brushes without having any? You can get brushes from friends, magazines and of course, online.
My favorite site to download brushes from is over at DeviantART. Once you've download your file, make sure you note where you saved the file to. Easiest to just download it straight to the desktop first. The file usually comes in one of two formats: *.abr, or any archived file, (*.zip, *.rar). If it's the latter, unarchive it first, its the *.abr file(s) that you want.
Step 2. Now that you have the brushes that you wish to install, its time to store it in a place where you will never lose it. I recommend you store it in Photoshop's default brush directory so you can find them again easily later on.
Locate your brushes that you downloaded earlier, copy those files (*.abr), navigate your way to your Photoshop folder. Look for a folder called "Presets", now look for the "Brushes" brushes folder inside the "Presets" folder.
Paste them into that folder. The folder structure generally is along the lines of: C:Program FilesAdobeAdobe Photoshop [version]PresetsBrushes
If Photoshop is currently open
Perform steps 3 and 4 if you have photoshop open, if its closed at this stage, goto step 5.
Step 3. Now that you have stored your brushes in an appropriate directory, its time to activate them inside Photoshop.
Select your Brush tool (b).
Click on the little down arrow next to the brush shape, a drop down menu should appear.
Click on the arrow pointing to the right, another menu appears.
Look down and click on "Replace Brushes...".
Step 4. Navigate your way to your brushes folder, it should by default already be in the correct folder, if not find it. C:Program FilesAdobeAdobe Photoshop [version]PresetsBrushes
Now look for the brush set that you downloaded.
Select the one you wish to load, and click "load". Your brush set is now loaded into Photoshop. Repeat Steps 3 and 4 for every brush set that you downloaded.
If Photoshop is not open
Step 5. Easiest step ever, open up photoshop. When photoshop opens up, the files will automatically load into the brush list.
Access the brush list by clicking on the down arrow next to the brush shape, and click on the arrow pointing toward the right, a menu should appear showing all the brushes that are in your default brush folder.
I hope this helped someone out there.