At your weight, including the advice above, I'd start with an upright hybrid, or even a mountain bike. The unfortunate equation is that you need to move your weight plus the bikes weight, and the bike isn't going to contribute much to that.

The one thing to really avoid though is the department store BSO (bike shaped object). It looks like a bike, it may ride like a bike, but it just isn't a good bike. Go to a real bike shop that sells brands like those listed above, and I'd include Giant, Cannondale, Felt, and a few others. DO NOT go to Walmart or Toys r Us or any similar store. You won't go wrong with a low priced Giant or Cannondale or Specialized or Trek. They make low end bikes that aren't crap. Also, don't buy a bike with suspension. Unless you want to spend a lot of money the suspension won't be any good.

My wife wanted an upright bike last year, so I spent a lot of time looking at higher end bikes (same brands as mentioned) and found what I thought would be the perfect bike for her, the mid-range Specialized Vita. When we went to the store, she ended up getting the low-end model of the Vita instead. Why? She tried them both and she could not tell the difference. The shifting was still smooth, the ride was the same, and it cost a lot less.