Wanting Advice about starting my first marine tank.

I have two freshwater tanks right now and I have no problem with freshwater what so ever. But I have wanted to do a marine tank but honestly I don't know a whole lot about it. The FOWLR tanks are awesome simply because of what you can get from just the rocks. I have a 20g tank that I thought about trying to start a small marine tank in and seeing how I would fair. Is 20g too small for live rock and fish? I know that it definitely couldn't be a lot but I am wanting some good advice to determine whether I can do that or if it would be best just getting something better. Also, I would like some real good tips on starting a solid marine tank. Thanks guys!