Hi--one newbie to another, here's what I've learned from these boards:
1) Most people seem to consider a skimmer essential, though some do operate without one. The general recommendation is to get the very best skimmer you can afford, preferably one rated for a larger tank than you are actually running.
2) Probably another powerhead; marine fish (and corals and etc) are generally used to lots of movement.
3) Lights--lots of options there, but no one seems to run the standard "came with my hood" strip lights. And--live rock!! 1 1/2-2 lbs per gallon.
Poke around the forums and you'll find lots of discussion of all these topics. Every time I think I have an original question, I search and find out that three other people have already asked it.
But I will say that this ain't a cheap hobby. I think numbers tossed around for startup costs are $20-$30 a gallon? I'm finding that to be just about right.
tank with 29-gallon sump
2x Maxijet powerheads
Mag drive 9 return
Coralife 4x65 Lunar Aqualight
120 lbs rock from liverocks.com
1 peppermint shrimp and 1 fire shrimp (very shy)
2 black Ocellaris, Squish and Smudge
3 chromis, too identical to name as yet, in QT
29-gallon with two goldies, Carrot and Orangehead