I don't know your weight, but at 390 pounds I picked up an Orbea road bike with a carbon fiber everything except the wheel set.

If you want to ride a road bike buy one, get it fitted and get a handlebar stem that puts you in a more upright position.

If you want a mountain bike, get one and have it fitted to you and get a stem that puts you in a more upright position

You can ride any style bike out there, just get it fitted to you.

I recommend the more upright position at first as it will be more comfortable to start.

I also would recommend looking at the bike stores for used bikes that would fit you and pay to have them tuned up and upgrade some of the components.

Start a new adventure and get a bike YOU CAN DO THIS!!