Good questions, Gr8GuyInFL. A lot of people avoid leg work and focus on the upper body. I'm not a trainer or a doctor, but I've read from a variety of sources that heavy leg work produces testosterone.
The Internet has many sites with videos or instructions on how to do specific exercises. You can also just get some of the magazines in the store or books with different exercises.
If you have bad knees, some can be very rough on them so get some good advice from a trainer or gym employee. You could also ask people to spot you and check your form (especially with squats).
I used to injure my lower back doing squats, until my trainer had me take the bar and stand right in front of the wall, while squatting. I had to keep my nose almost touching the wall. Once I learned to do that, I stopped injuring myself.
Here are some basic leg exercises I like:
1) Squats (you can start w/Smith Machine)
2) Hack Squats
3) Stiff-Legged Deadlifts
4) Lunges (I love walking lunges)
5) Leg curls
6) Leg Raises
7) Calf Raises (seated or standing)
The best motivation for me is looking at guys in shorts who DON'T work on their legs but have great upper bodies. Their bodies look very out of proportion, especially if they take steroids and don't work the lower body. However, some men have great difficulty increasing muscle mass in their legs, especially calves, no matter how much they work out so don't be quick to judge.
Strengthening shoulders and back will help your posture and your ability to lift when doing other exercises.