Hi welcome to the site.
Lighting very much depends on what corals you are going to keep? For instance if you want mainly soft corals and easy to keep corals you could get away with T5's or metal halides. Halides are expensive to run however and can get hot meaning they will potentially heat your water. Many people go for LED
's these days (myself being one of them) as they are cheaper to run than other lights and create amazing colours in your aquarium. The initial cost can be quite high though.
Heater..... Get a good quality one. I use Rena heaters and they've never let me down. I steer clear of glass heaters as these have gone very wrong in the past in my experience.
Are you testing you water? If so what are your parameters? I'm guessing after only 1 week your tank isn'y cycled yet.
As where fish are concerned its a litte limited in a 15g however there's a few little ones you can go with. The Royal Gramma would be my favourite. Just do your research on the fish befopre you buy it. Even post on here if you have your eyes on one and we'll help you make the right decision.
Congrats and would love to see a few pics of the progress!