[lmm]

The game ends when both players pass. That's very simple and clear.

You score by counting the number of unoccupied points in your territory and adding the number of your opponent's stones you captured, + the "komi" that the second player gets to make up for going second (usually 6.5 for a full size board). (A common technique is to place all the stones you captured on empty points in your opponent's territory, so that you're subtracting the number of your captures from their territory). The only possible ambiguity there is around what constitutes territory; if you like then as a beginner you can just make the rule "unoccupied points in regions surrounded entirely by stones of your colour", which will make for a few tedious turns at the end where you fill in the last few gaps but won't change the scoring at all.