To start you'll need:
Baisc understanding of cycling process
Chemicals to test ammonia, nitrate, nitrite
Live rock (lbs dpends on what size tank you get)
Live sand (if you dont want to get a full live sand bottom, you can use argonite sand and seed it with a cup of live sand or let your live rock seed it.)
Filter (depends what size your tank is)
Heater (depends what size your tank is)
Lights (depends what kind of corals you want)
Power heads (amount depends on the size of your tank)
Nanos are apparently harder to keep, since you need to keep up with your tank maitenence and what not. Meaning 10%^ water changes every week, scrubbing algae, testing regularly for ammonia spikes and other spikes ect. Since you have less of a volume of water, you have less of a margin for error. Dont let that scare you, if you think you can keep up with the work you'll be fine. I have a 10g nano tank that has been up for around 6 months now. This is also my first saltwater tank and everything seems to be going well, so yes it is difficult and expensive, but it can be done.
The best thing to do is look at other peoples nanos, research, observe and ask lots of questions on everything you dont understand.
Then delevope a plan on how you want your tank to look and function including your stock list and compatibility of the fish inverts and corals.
Then start with adding your live rock and sand to the tank, arrange it properly so you dont have to keep arranging it. Live rock goes in first on bare glass, then live sand (this is so if you get a fish that diggs you wont have a rockslide). Also before doing this make sure you have your necessary equipment installed (heaters, filters, power heads ect.)
Then you wait for your cycle to complete. A cycle is a period you wait for benifitial bacteria to grow to take care of breaking down ammonia into nitrites, then nitrites into nitrates. Nitrates are accpeteble at a low concentration, and are considered less toxic for marine life then ammonia and nitrites(but you still want to try your hardest to get rid of all of the traces.)
You test using your chemical test kits, wait and once the cycle is complete(you'll see an ammonia spike, then itll go down to 0, then a nitrite spike, then itll go down to 0, then a nitrate spike, then it should go down 0- something I forget (it should say on the test kit)) then you add your clean up crew depending on how much algae growth you have, then your fish, then your corals(depepends on the lighting you want)
When you start to add corals then you'll need to buy a reef test kit to test for your calcium levels and other chemicals that your corals need to survive in the water.
Thats a quick way to describe things! But I stress, if everything seems like its a lot to handle YOU NEED TO RESEARCH MORE AND READ MORE ABOUT THESE THINGS BEFORE STARTING!
go slow take your time, read, read, read, ask questions, read some more then purchase!
now all i need to do is get the tank and begin, and while its cycleing continue researching