Contrary to popular belief, piranhas are actually pretty boring to keep in a tank. They spend most of their time "sitting" and not moving. Oscars are also a more advanced fish to keep. They sell lots of little babies in fish stores to unsuspecting owners. You can always keep one of these options later, but in the meantime I suggest you start with something more basic in your tank until you get the hang of maintaining it. If you want an interesting semi aggressive tank you have tons of options, but let me warn you that having to fish out dead fish really is no fun.
Here is my suggestion: 5-7 tiger barbs, 1 gourami (any color), and a crayfish on the bottom. This tank would be full of color, active, and predatory as the crayfish is a tough little bugger that will sometimes catch fish and eat them. You could also replace the tiger barbs with the black skirts if you wanted.
Of course, do NOT add ANY fish to your tank before you are familiar with cycling. Take a look at the link in my signature for some good information on setting up your tank for success.
