I'm 29, overweight, and want to start biking to get some of it off.

I live in the Northwest suburbs of Chicago so the weather varies lot but I doubt I'll be riding a lot in the rain. When it does rain here, it pours but it passes quickly.

I want to start out by riding to and from work (4 miles), and then branch out from there.

What type of bike should I look for to start out with? I'm guessing I need something that is generic for most purposes?

It's very suburban out here so it would be mostly sidewalks/side of the road biking. It's mostly flat terrain but where there are hills they're steep. We live very close to a forest preserve with a bike trail that I'd like to ride on also.