Dont start this hobby...find a magazine, cut out a picture of a nice looking reef tank...and tape it to the fron of your FW tank...it will save you tons of cash, headache, frustration...oh and did I mention cash and headaches?
If you are insistent...then do what BangGuy says...except a couple of different books by a couple of different people will give you a couple of different perspectives...
Search online, here, everywhere, read, read, and then go ahead and read some more prior to even starting.
I think besides a few process (denitrification), gills, and water...they are two completely different monsters (FW vs. SW)
Good luck, tank some tylenol...and enjoy!
Oh, and did I mention reading?