It is normally better to buy aquarium equipment online, most of the time it is 50% less than at the
lfs. Aquariums are better bought at the
lfs though. They are harder to ship and by the time you pay shipping, it would have been cheaper just to buy it locally.
I agree with the others, bigger is better. You could start with a 10
gal, but your stocking options would be limited. You're going to come across so many fish that you're going to want to keep and smaller tanks can not hold them. Plus, larger tanks are easier to maintain because of the increased amount of water. 55
gal tanks can hold almost any type of fish that you want to purchase. Now bare in mind that there are some fish like Pacu's and the such that can not be kept in these tanks, but most fish will. And if you can afford a 55
gal tank, like Meredith said, go for the 75. They are not much more expensive.
Welcome to AA!