Welcome to the addictive hobby!
My best suggestion would to buy a book and take a week or so and just read it.
My first book when I was converting a fish only to a reef was the Moderm Marine Aquarist, and definately helped me set-up the tank properly. It explains the cycle process, and covers a lot of other items, lighting, filtration etc. It's an easy read.
More in depth would be the Conscientious Marine Aquariast.
Top answer your questions a little more, what you do and how you do it really depends on where you want to end up. If you want only fish and live rock (FOWLR) then your filtration and lighting requirments are different than if you want a reef tank (fish and coral). Fish are generally more tolerant of water parameters than coral, so when you start introducing coral to a tank you need better lighting, better filtration, and less fish. For salt water fish the undergravel filter is sketchy at best. For fish and coral, your filter will definately not work.
It is definately worth taking some time and trying to deide where you want to be. As it will take your tank about 4-6 weeks to cycle, you have tons of time to think about it!
Read and go slowly would be the best advice I could give, and it was the best advice I received when I started my tank. If you are making choices about equipment and you are unsure, then I would pick the better alternative...ie - I would dump the undergravel filter and get a skimmer instead, that way your choices are still wide open. Whereas if you go with the undergravel filter, you will not be able to have a full-blown reef tank.
There are some generalities in this advice, but it is roughly accurate. Read one of the suggested boooks, and you should be in a good position to make your own choices.
Good luck, and we are here to help.
Stewart
