Before diving into the aquarium hobby, consider which size setup is right for you based on:
How many fish you desire, and how large they become as adults. It is recommended to have 1 inch of fish per gallon of water so that they are comfortable, have swimming space, and have enough territorial space to remain peaceful with each other.
Will you have a reefscape with live rock? If so, keep in mind that the recommended 1-2 pounds of live rock per gallon of water will take up much of your aquarium space. This means you will either need a larger tank for a desired fish quantity, or a call for less fish in your tank.
In the future, do you plan to move to a larger house or gain a higher income that could be spent on aquarium equipment? If so, think ahead. Right now you may not want to spend very much money on the aquarium hobby, and might not have too much space for a fish tank. However, consider future plans because it might not be worth it, for example, to buy a 10 gallon fish tank now and then want a 50 gallon tank later. This would end up with you buying two tanks, and needing the proper equipment for both; spending more money than if you planned ahead.
If you want an aquarium for a bedroom or desk, consider getting a nice nano aquarium, betta tank, or small 10 gallon aquarium. These would be cheaper, easier to move, and easier to care for options than their larger tank counterpart.
To learn about some great, high quality fish tank choices, consider the following setups for: