What do you want to do? Are you thinking of road riding? Go to the road forum and check the under $700 bike thread and then do a search for "Fit", as in fitting a bike to your body. There are lots of threads about training, but if you haven't been on a bike for years, the best thing is to just start with short rides at your own pace and have fun.
If you are over 50, or close, the 50+ forum has a lot of good information, too.
You could also find a good used bike for cheap if you know what to look for.