Is a bicycle the right fit for you if the ball of your foot is touching the ground?

The basics of bicycle sizing is that when you straddle the frame(Not the saddle!) you should have some clearance between the top tube and your crotch. When you're in the saddle, with the ball of your foot on the pedal, then you should have a small amount of bend in your knee with the pedal in its lowest position.

Then there are the niceties like crank arm length, and reach between saddle and bar, etc etc. Run a netsearch on "bicycle sizing" and you'll find some more detailed pointers.

Keep in mind that if you're looking at bikes which generally aren't ridden seated, like BMXes and DH MTBs, these fitting basics don't work particlularly well any more.