Both forms are correct.
"Встань" - is for "[you,] stand up!" - it's the imperative form. "Встать!" is the infinitive form that also can be used in modern Russian as an imperative. The difference is that infinitive imperative is more command-like, hard and generally speaking more offensive.
A policemen can shout out: "Встать!", but mother would tell her son: "Встань". A lover can ask: "Смотри мне в глаза", a detective can yell at the suspect: "В глаза мне смотреть!".
The other difference is the
встань is for a singular you ("ты" in Russian), for referring group of people - or just being polite - one should use
встаньте. "Встать" is correct both for "ты" and "вы".
UPD: as of what's the difference between
вставать - this is perfectly covered here.