Under the Fixed Term Parliaments Act the government can't just call an election.
So, May needs to either:
1) No Confidence herself. And then wait 14 days. Easy to do, even if it looks a bit silly.
2) Get Labour to go along with the snap election. Which they'd be stupid to do, but will probably do so anyway, considering recent behaviour.
2) Repeal the Fixed Term Parliaments Act. Would probably take a while, particularly if the Lords decide to bounce it around.
I wonder which one she'll go for. By what's been said so far, it looks like (2), but it's hard to tell.
Original post on Dreamwidth - there are comments there.