As in many languages, you gotta learn to differentiate between polite and casual ways of talking...
If you're going for an absolutely polite form, you could use
'Disculpe, cual es su nombre?'
(literally 'excuse me, what is your[polite] name?)
tu/su mean the same, with 'su' being a polite form, which you should use when you're talking to someone you owe respect, or don't want to sound rude to.
About 'tu', keep in mind, 'tu' means 'your', and 'tú' (see the 'tilde' mark) means 'you', as in a pronoun.
If you're going for a more casual style, any of the above work.
We supposedly have both ¿ and ? signs, but unless you're writing a report, or for a diary, the opening mark is pretty much omited. A comma wouldn't hurt though.
Hola, cuál es tu nombre?
(watch the tilde again. for this word is not completely needed, but it's a good practice to at least keep it in mind. literally: Hi, what is[casual] your name?)
'Hola, como te llamas?'
(literally, 'Hello, how are you called?')
Hope it helps