One thing to keep in mind with salt water is that the smaller the tank the more work it will be to keep it healthy. The larger the tank, the easier it is to care for/less work involved, and more space for animals you may desire. It is quite often that once someone begins in the saltwater hobby, a single fish in a tank is simply not enough and more are desired. 10 gallons would limit you to 1 fish, as Pasfur has already said... and the selection for a 10 gallon tank is quite small.
One more thing to consider... if wishing to keep an anemone with a clownfish or a pair of clownfish with or without anemone, it would require larger than 10 gallons... Specific species of clowns will bond with specific species of anemone. In the case of oscellaris it would be a bubble anemone, which grows quite large. You may want to do some research on the various species of clownfish and the anemones who host them. Anemone sizes can be extreme for some species, such as ritteri, sebae, etc.
I agree with 1 single oscellaris clown or 1 of the species of damsels that Pasfur mentioned, but I have seen the yellow tail damsels reach sizes that would warrant more than 10 gallons. (up to 6 inches in length as mature adults). 1 maroon or tomato clown could do well in 30 gallons, which is what I tend to suggest also for the yellow tail damsels if kept singly. These fish have long life spans, and it is not out of the ordinary to expect 10 or more years with a clownfish. This is a long term commitment.
Are you wanting to add corals to this tank or keep it a fish only set up? That should also be considered when determining how large of a tank you will want to start with. Many species of corals are not compatible with each other in small tanks such as 10 gallons, so the smaller the tank the smaller your selection of compatible inverts as well.