Visit the local bike shops, Trek rates their mountain bikes at 300 lbs but most quality bikes will hold that much. The most important thing is to get the correct size frame so it will be comfortable to ride. You want a hard tail mountain bike, comfort bike or a hybrid of the two. Forks with a lock out feature will help you climb hills and save energy when they are not needed. Start out slow and ride for distance not speed and you will enjoy it a lot more. It will take several weeks to get to the point where you are burning a bunch of calories. Diet is very important because you can out eat any exercise you do.