I'm a big fan of walking and other informal exercise: gardening, housework, etc (always been careful of your back, of course).

I'm not so enthusiastic about trying to learn weight training on your own. It's critical to keep good form when lifting weights and it is so helpful (honestly, I'd say essential) to have someone knowledgable about weight training, preferably a trainer, watch you and coach you on proper form as you are starting out. The Y is a great idea if you have one nearby.