You can do anything you want to, but I strongly suggest something larger than a 10 gallon as your first saltwater tank. Freshwater and saltwater are very different in the demands of the fish keeper, and the needs of the marine animals.
Although similar to freshwater in some respects -saltwater chemistry and fluctuations if they get out of whack, can easily lead to tank failure, loss of animal life and a discouraged hobbyiest.
If money is tight - I would at least try and save up for a 30 gallon minimum, read up on the hobby as your are saving cash, watch your local trading paper for folks wanting to sell their used stuff, and start accumulating what you need when you see it on sale.
As Rapt0r32 said - a 55 is an ideal size first tank, and there are always people wanting to sell their stuff. You'd be surprised on what you can find out there - at a good deal.
Experienced hobbyiests that set up and run small 10 gallon tanks or smaller, do so successfully because they have already mastered the basics.
Little tanks are tough - ask anyone.
Starting out with a 10 I'm afraid too often leads to an unsucessful experience.
You'll want a larger tank 2-1/2 minutes after you set up the 10 gallon anyway
Wish you luck Fishey