When I started to first ride...just months ago I was 344 pounds I bought a used trek 1989 Steel Lugged frame mtb it has the double wall rims and I bought the longer stem and even more upright handle bars with a 3.25 rise and it was great I found the bike on craigslist for 100 bucks just to make sure I did not spend a lot on the first bike as it was being experimental..meaning will I stick with it. I am sticking with it, and I have since bought a giant steel frame sedona st bike comfort bike cause I am not a mtb biker just like to ride for exercise and enjoyment
good luck to you
Like most have said get a bike with a solid front fork and solid seatpost too cause a suspension post will not work