Well, first off I would figure out what types of fish you want in your aquarium. Some fish can do fine in smaller tanks and others need a lot more room. Once you narrow down what fish you want, you can go from there. I started out with a 10 gallon, but quickly realized I needed to go bigger. My largest tank right now is only a 30 gallon, but I would love to go bigger when I have the space. So that's another factor, where are you putting this tank and what kind of room do you have?
Do you plan on having a planted aquarium? If so, you'll need a bulb or light fixture that can support plants. 6500k is a rule of thumb. I'm only familiar with fluorescent lighting, so you'll have to consult someone else if you plan on getting into LEDs. Also for plants, you need to decide on a substrate. Some people stick with just the general aquarium gravel and they have great results, other people go for soil and sand.
As for the filter being internal, could you possibly have a HOB (hang on back) filter? That's what many people, myself included, use. I know people use sponge filters internally, but I don't have any experience with them. But that's another option to look into.
I would also definitely invest in API's Master Test Kit
for measuring pH, ammonia, nitrates and nitrites. It is invaluable, especially when you start out. When starting out, you need to make sure your tank is cycled, for a great sticky on all that is cycling, check out this link: http://www.tropicalfishkeeping.com/...nners-guide-freshwater-aquarium-cycle-353074/
Hmm, I think that's a good general start.