you need to consider what type of tank you want...or what you want in it i should say...i started with a 10gallon, and moved to a 20gallon long, and in time will prolly get another bigger tank, but a 20 will be fine, but things you can get are more limite....some aggressive fish will need bigger tanks, like tangs, triggers,etc.
Smaller tanks MAY be harder to control the water parameters, but ive never had that hard of time....so decide what you want, then decide on your tank....but fair warning, the hobby is addictive, and you'll probably want to upgrade anyways down the road!! The choice is yours!
ENJOY!!!
Luke