The single biggest factor in whether a bike feels right isn't the brand, the gears, or the colour — it's frame size. A bike that's too big or too small will never handle well, no matter how good the components are.
Frame size is usually measured by standover height: the clearance between your body and the top tube when you stand over the bike with both feet flat on the ground. As a rough starting point, you want 2-4cm of clearance for a road-style frame and a bit more, 4-6cm, for a mountain or hybrid frame with a sloped top tube.
Standover height gets you in the right ballpark, but reach and saddle height matter just as much for comfort on longer rides. With the saddle at the right height, your knee should have a slight bend at the bottom of the pedal stroke — a leg that's almost fully straight or still noticeably bent means the saddle needs adjusting before you decide the frame itself doesn't fit.
Kids' bikes are sized differently again, usually by wheel size (16", 20", 24") matched to age and height rather than standover clearance, since a growing rider will size up again within a year or two regardless.
If you're between two sizes, or unsure after checking these numbers at home, that's exactly what an in-person fit check at the shop is for — a five-minute standover and saddle-height check catches most sizing mistakes before you've paid for the bike.