MTBR is a great source of information, but it has its limitations. Any mountain biker, in the flesh, could have (and in the future, could) help you get a fit like 75% as good as the most highly trained professional fitter could do. Frame size, seat height. Done. 5 minutes in real life. Brake lever position, 1 additional minute. Or dozens of posts and photographs and measurements and calculations over the net.
Even just to make fairly simple choices as a consumer (for instance, buying a bicycle), really takes some expertise. Some of this stuff is so basic that long-time riders might not even realize it is expertise, but when you don't have it on your side....
Maybe you just moved to a new city and don't know anyone; maybe you are the only person in your whole circle of relations that rides a bike; even then, you are allowed to go 'use' the knowledge and professional friendliness of the salespeople at the local bike shop. They would prefer that you then buy from them, but you're not obligated. They should realize that often someone brand new to biking is going to try to spend less than even the least expensive bike in the shop, and that they are doing the right thing to treat you well and to grow the sport, and win your business in the future. Try not to abuse it, and virtually anyone and everyone in cycling will try to help you out at least a little. Good luck!