Well, step 1 is to determine where your nearest VA health center is. You can just use Google maps and search for VA Health Center. You can tell me your city and I can find the nearest one if you want.
Once you do, go to their website and find their Registration Office phone number. You'll have to get registered before doing anything else. It's a pretty painless process (relative to trying to renew your CAC card at DEERS), just need your DD-214.
When you setup your appointment to get registered, ask them to give you the phone number for the disability rating office. They'll be able to walk you through the process from there. Generally, you'll want to have all your paperwork in order before showing up for your appointment, especially your medical records from your service. Oh, and make photo copies. The form you'll have to fill out is here:http://www.vba.va.gov/pubs/forms/vba-21-526-are.pdf
You'll get an appointment date for your evaluation, and depending on what you claim, you might have a few appointments to go to. It will likely be a few weeks, but don't get discouraged. Make sure you tell them EVERYTHING regarding medical claims. Even if you don't have paperwork to support it. Even if your knees don't seem that bad now, you have to imagine yourself in 20-30 years when trying to make a claim with the VA will be all but impossible. Get it all on record now to save you a lot of hassle later in life.
Another tip is to see if there are any veteran organizations in your area, like a VFW or AmLegion, as they can help you with the VA.