WELCOME
While there are people who keep very small tanks with success, they are generally not for a beginner. With the small water volume, it is very difficult to maintain proper water conditions and there is no room for error. I do not believe there is a saltwater fish small enough to do well in only 5g. The bio-load that a fish produces could definately foul a 5g very quickly. Tanks this size are usually set up for inverts only.
For a beginner with a new tank, I recommend you start no smaller than 29g (preferably over 50g). Your chances of success would be much greater, and you'd have a much wider selection of fish you could keep.
:happyfish