We elect the House of Commons and the rest gets worked out from that. The UK is divided into 650 constituencies (also called seats) and each one elects an MP to sit in the Commons by "first past the post" - whoever gets most votes wins. Effectively you've got 650 separate elections going on, and you only get to vote in your local one to elect your MP.
Once the counting is done and we know who all the MPs are, and therefore how many each party has, then we know the next bit. If a party gets more than half the seats, it can outvote the rest in the Commons so it can form a stable government and get new laws passed, and its leader becomes Prime Minister. Simple as that.
But if nobody "wins", no party can do that and they'll be thinking about doing deals to work together so that together, they've got the magic 326 seats or more. If they make a formal agreement to form a government together, that's a coalition. That happened last time, with the Conservatives and Liberal Democrats. Or the party with the most MPs could try to go it alone as a minority government, perhaps with the agreement of a smaller party or parties to vote with them and keep the government going.
At the moment, as I write this, it looks like the Conservatives will JUST about get the magic number and be able to form a government on their own.
Franky, it is a different system than America. Google How the Election Process works in the United Kingdom- For Dummies and there is a brief 6 step explanation. A party has to get more than 50% of the 650 seats or 326 to be in power. If they don't get that many they have to make deals with other political parties to form a coalition to get those 326 seats. Ask Grandma- she will give you a simple answer.
One party needs to have more seats than all the others added together to be able to pass laws and be sure to get enough votes in parliament to make them legal. If not, they may have to grovel for support from other parties.
UNFAIRLY....MILLIONS can vote for a given party and see next to NO SEATS.....
