I agree with Bike Spokesman. Here's a couple exercises I like. Sit on a pad on the floor in front of a exercise station. Bend the legs a little so you can row. When you row, try and keep the back straight. Bend from the hips, and make sure your abs are tight on the return swing. That's a good warm up exercise for the back. After you can do that at 80-90 pounds start in on the Roman Chair. I really like to hold a weight against my chest when I do that; but add back work gradually. Once a week is plenty, Once every 5 days at the most. Another thing I like is Ascending Pyramid sets.
You can vary this to suit yourself; but the idea is to start easy; and add weights and reps slowly. You can vary this, but do 2 reps at 20% max, 4 at 40%, 6 at 60%, 8 at 80% , 9 at 90%. The idea is to pump the muscle up; not to go to a burn.Like with cycling, you want to mix it up. With the pyramid you only hit 100% every third or fourth workout. You then start over and add 5 or 10 pounds. So you might try doing a high rep workout no higher than 60%. Then the next time could be 80% with the standard 3 sets of 10. Then on the 100% day use few reps. Say 1 at 40%, 2 at 60%, 3 at 80%, and then as many as you can at 100% (but not more than 12). Some of the traditonal weight lifting exercises are hard on the joints or back. Lunges are an example. It's easy to work any of those muscles without using the knee in such an unnatural manner. And speaking of knees, learn where the Vastus Medialus is. It's a knee muscle, it works on the upper part of a Leg Extension exercise, roughly 70-90% of range of motion. You don't want to go all the way up so the leg is fully extended. Finish with a set or two where you just slowly go up and down from about 3/4 of the way up, to about 9/10 of the full extension. You can feel the muscle working above, and to the inside of the knee on the thigh. That muscle protects the knee. Keep it strong and you can avoid a lot of grief; it's also a killer exercise for skiers.