Summarising the comments in a community wiki
The fixed term Parliament act does not allow for the Prime Minister to call a general election without first consulting Parliament. If a Prime Minister wished to call a general election before the set period of 5 years, she would have four options:
Attempt to pass a resolution through commons calling a general election. This would require a two-thirds supermajority, so would normally require the support of opposition parties. (This may happen if the opposition feel that the damage they would suffer from refusing an election, and appearing "scared of the electorate" is worse than the damage they would suffer from going to the polls early)
Pass a motion of no-confidence in her own government. This would be extremely unorthodox and would leave her open to criticism that she was "gaming" the system.
Get parliament to repeal the Fixed-term parliaments act. This would be normal repeal bill, and so would be subject to a simple majority in the commons and lords, but could be delayed by the lords, in the normal way.
Pass a one-line bill which states "notwithstanding the fixed term parliament act, there will be an election". This would only need a simple majority to pass, but would have to be approved by both houses of parliament.
The first option seems the most likely, but this has never been tried.