How does a state determine how many electoral votes a candidate gets?

Each state is awarded the same number of electors as its total of senators ( always two) and its number of Congressmen ( determined by the state's population) When people may think they are voting for a president, they are actually voting to choose that state's electors. The candidates for electors are chosen by their parties and pledge in advance to vote for their party's nominee.
Most state have a winner-take-all system-- the party with the most votes elects all of its electors and they all vote the same way in the electoral college. Two states split their electors according to the split of the votes.