There are some good recommendations in here. First off - check this site or another like it:
Bike Fit - Proper Bike Fit Can Prevent Pain and Injury
Next - as mentioned - biking is vastly different from doing squats or leg presses. One will help the other though.....I can attest to that. Biking is 90% of my cardio, I bike anywhere from 30-60 miles a week on average.
Learn to utilize the gears to your advantage depending on the terrain you're on as well, ---- if you're riding for pleasure, it should be easy and comfortable, and you should be coasting quite a bit. If you are riding for a workout - it should be constant peddling, and you should be feeling resistance at all times.
Road bike or mountain bike?.............that matters too.