You need to find a good bike shop that caters for non athletic riders. Its your money and they want it so they should treat you properly.
The overweight aspect does not really impinge on bike selection, all bikes are going to be strong enough. You do need one which is small enough to ride comfortably, a good shop will help you select and fit a bike. They should also check the assembly of the bike, fit any accessories and upgrades you select and provide after sales service such as a tuneup after a few weeks use.
Avoid discount $99 junk bikes. Go for a respected brand and start with a budget of at least $200. At the entry level end, a little more money buys a lot more bike. You will have to budget for a few extras inc helmet, gloves, puncture kit, waterbottle etc. You can use these accessories for negotiation rather than cash discount.
As for the style of bike, small riders are better served by small wheels, in your case stick to 26" MTB size wheels. MTB or "comfort" style bikes are fine for riding trails.