Lots of people have mtb in their garages they never ride. You can easily find a decent used bike at a garage sale, on Craigslist, Goodwill, or at the local bicycle cooperative. It's a good idea to start with a cheap or free used bike after you've spent some time researching the right frame size to look for. The only thing that really matters is fit. The parts/components on real-brand bikes are all good. Sometimes you can find a bike that's barely been ridden. In my opinion a suspension isn't necessary, but if you must have one a suspended front fork is all you really need. I like to ride rigid bikes and love their reduced maintenance. I recently found a nice used mtb neighbors had put out on the curb and am liking it. My city has a non-profit bicycle cooperative so I go there to find parts. Use a helmet and buy clip-in mtb pedals and shoes. The mtb pedals/shoes make a big difference in pedaling efficiency and you can very quickly release your feet with a twist motion. It seems to me the Walmart type bikes run on the small side and unless you are small it's probably better to find a used real-brand bike.