First things first, you should go to a physiotherapist or an osteopath to get your back sorted out now. If you compromise certain muscles groups to avoid the pain of the strained muscles, you'll end up with other things hurting from doing that!
Also, take some ibuprofen or other anti-inflammatories (ask at the pharmacy) to aid the healing process.
Pilates and yoga both have exercises which specifically aim to strengthen the back muscles. Other than that you can do weights on the machines at a gym (make sure you get an instructor to show you how to do it properly before you try it, otherwise you risk injury).
Swimming is also excellent for stretching AND strengthening the back at the same time.
Some really basic exercises to stretch your back before you use it: (more in-the-know people will probably have some better ones)
*Standing up, bend over and bend your knees a tiny bit so that you can clasp your hands behind your knees, and use the weight of your legs with your hands holding on them to pull and arch your upper back, and you'll feel the upper and mid-back stretching the whole way across.
*Kneel down and sit on your calves so your legs are totally bent back under your bum, and lean forward right down to the ground and drop your head down to the floor, and reach your hands right out as far as you can reach. Feel the stretch in your upper and mid-back.
*There's also the "cat stretch", stand like a cat on all fours (i.e. knees and hands on the ground with your feet out the back), arch your back up and tuck your tummy in to stretch the back, then flex your back the other way so it makes a concave shape in the middle of your back. Man that is harder to describe than I thought...does that make sense?
*Lie down flat on your back, put your right hand/arm out flat on the ground so it's perpendicular to your body, then bring your right knee up so it's at a right-angle and keep your foot on the floor. Then with your left hand pull your bent knee over towards your left side, while keeping your face pointing up to the ceiling. Repeat on the left side. I love this one because it stretches your bum, lower back and thighs.
Let me know if you need me to find pictures to illustrate any of those :D