Tried any in your hand? Shot any? Got a gun range that rents? That'd be your best bet to start.
Budget?
Single action with manual safety? (1911/Browning HP pattern)
DA/SA with decock or manual safety? (CZ, Sig 226 etc etc)
DAO with no manual safety? (double action only)
Partially tensioned striker fired, no manual safety... kind of a long single action? (Glock, SA XD, S&W M&P, many others)
Size of gun? (Full, compact, sub-compact)
Single stack, double stack magazine?
All steel, aluminum frame, polymer frame?
Weight an issue?
Intended holster? (That's a thread unto itself, inside or outside the waistband for starters)
Fully adjustable, fixed sights or a derivative like the Colt channel sight?
Right or left handed? (Some guns don't lend themselves well to lefties)
Revolver... we sort of start over with the revolver variables, which are fewer.
There are other aspects to consider... these are but a few of the more common.
Cheers,
C