I too sort of recently started getting into marine tanks. Marine tanks can become overstocked easier than freshwater. They also require more work.(it water changes are longer/harder with SG in the equation).
Live rock is VERY important. It is recommended to have at least 1 pound of live rock per gallon of water. I am not sure if you can have to much, Most things I read say between 1 and 1.75 lbs per gallon.
Most people recommend 1 inch of fish per 5 gallons of water. You can get away with a little more then that, but the more you push it, the more you need to watch your water quality.
Lighting is also very important as said above. Corals and plants especially need good lighting.
As for getting corals, you will need 'reef-safe' fish; this means the fish most likely won't hurt/bite your corals and other inverts. Check out this site, it is where I purchase most my fish now(
www.liveaquaria.com). Their fish are labeled as 'reef-safe' or not. You can get some good ideas. They also have a marine compatibility chart so you can see what fish are ok together and which aren't. So having corals will limit your fish. My tank I can't put corals in because of the fish I have
but that is why I started a nano reef
One 'cool predator' fish that i feel partially 'reef' safe is snowflake eels. Either the black "freshwater" eel which isn't truly a freshwater eel but a brackish eel that can live in SW. Also the snowflake eel. You can put eels with corals and other things like feather dusters(at least I have and they were fine). The eels also leave alone my urchins and seastars. But you throw in any type of crab(hermit, red claw) or crayfish and they will get eaten immediately. My eels also don't touch any of my fish. My 'FW" eel is over 17 inches long and in over 3 months hasn't touched any of my fish(some as small as 2" including half black angelfish, green mandarin, scooter bleenie, and others). I feed my eels mainly cocktail shrimp.
Sumps/Refugiums come into play also with SW tanks. They are purposefull on FW tanks also(I use them) but most people use them mainly for saltwater. I have some videos of my setups and some explaining refugiums/sumps/overflow boxes. Check them out at
www.youtube.com/earthling1984.
If you have any questions, feel free to ask here or pm me