Welcome.
Go slow. Your tank will need to cycle for at least a month to be safe. Read everything you can, here and on other sites like wetwebmedia.com and in reference books. Be a sponge - absorb info, but make up your own mind.
Try not to get too excited at the LFS (local fish store) because you might be tempted to by something which you know nothing about. Remember, LFS are businesses, in the business to SELL you stuff. Most good shops will allow you to put it on hold (maybe with a small deposit - but make sure its refundable), so you can go online or to your books to look stuff up.
Remember too - most creatures in this hobby (fish, corals, inverts) are beautiful and interesting, many are impossible to keep & many are incompatible.
Find out adult size, because with only a 27g, you're going to only be able to safely keep a small handful of 2 inch fish or a couple of 3-4 inchers.
Decide now whether you're going to do a reef, a fish only with live rock (FOWLER), invert, or species tank. Then you can plan your investment & equipment to suit their individual needs. Reef's need excellent water quality & lighting. Fish are more forgiving and better for beginners. Both can get expensive very quickly (especially corals).
Bring your water in to be tested at the LFS periodically, especially before adding your fish or corals.
Don't be shy to ask questions. Lots of info out there, just have to pose the questions.
Remember, Marine tanks cannot be stocked like FW. Examine every potential purchase carefully for signs of damage (torn or missing flesh on corals, white spots, discolouration, lesions, heavy breathing with fish). Don't buy it if it doesn't look healthy - it's most likely on its way out. Regardless of all your precautions, be prepared to take some losses. If you are faint of heart, get out NOW
Once it becomes an addiction, your 90g will seem to small & you'll wonder why your wallet is always empty & your VISA is always worn out.
Sorry for the long post.
Good luck. Hope this & other postings help
Anthony