There's no one best way. Everybody has different interests.
First, of course, is to get a bike. Do you have a bike? Your local bike shop can help with recommendations based on how you plan to ride and what your budget is. Test ride before you buy. With your back issues, it is extremely important that your bike fit you well. So before starting to ride, either have your existing bike professionally fitted or get a new bike with a professional fitting. If you have unusual medical issues, you may need a sports-medicine clinic with a trained bike fitter. Otherwise, you may be able to use a local bike shop.
Once you get the bike, then all you have to do is ride it. Start slowly with short distances, back off a bit (but don't stop completely) if you have pains. Some pains are expected and need to be worked through (e.g., most new cyclists experience some butt and back pain at the beginning, and muscle soreness is a given). Find places where you can ride comfortably and safely. Maybe you have a good bike path that isn't too clogged with pedestrians. Maybe you have a lonely country road nearby. Buy whatever clothes you need to ride comfortably. If your state produces a bicycling manual and/or a bike map, get copies. Use a helmet. Follow the rules of the road (e.g., stop at stop signs and lights). Be courteous and alert. Don't listen to music or headphones.
Note: To head off flame wars about helmets, ipods, stop signs, etc., let me say this. My recommenations are to a beginner. Once you gain experience, you can make your own informed decisions about these things.