Welcome to the board. If you're just starting out like I was last december, red belly piranhas are probably the best to start out with, being widely available, and somewhat cheap. But in order to be entertaining, at least 3? for a small schoal would be best, 1 in a tank is shy and boring to watch. and 2 fight with each other. I'll let somebody else who has more expirience get more into detail on that.
welcome, you have quite a selection and if you decide you want some other cool fish like arowanas or oscars or something we have a nonpiranha forum either way, even if you have no piranhas you are still welcome to the site and you can always get some in the future, it took me a couple months to get my reds.
Neon tetras are a nice, docile starter fish for a beginner like yourself, but you'd need a 900 gallon tank setup to hold just one, as they can easily exceed 3 feet in length. They're a beautiful bright red/neon blue/pearl white color, and a shoal of them is truly a sight to behold. Seemings how they are a schooling fish, however, you may consider creating a 2500+ gallon indoor pond of some sort, to make them happiest.