A bike fits you when you have plenty of clearance between the horizontal tube and your crotch, and when you can reach the bars without stretching or hunching up. How far you lean forward is up to you, racers like to get really low, beginners tend to like riding with a very upright back. The best position for comfort and control is probably in between these extremes.
"Comfort bikes" tend to favour a very upright riding position. They also have heavy frames and suspension system. You dont need this and it adds weight.
Cheaper (sub $600) mountain bikes and so-called hybrid bikes generally avoid sporty riding positions, and seem OK for general riding about town. They have wheel rims which take a variety of tyre widths. A medium sized smooth tyre is best, but any good bike shop can change them before they sell you a bike. Its not something to worry about yet.