The most important thing to remember is this:

If the verb has a 'd' or a 't' sound before 'ed' you need to pronounce 'id'. If it doesn't, don't pronounce an extra syllable.

Be careful! It's the sound, not the letter: 'decide' is pronounced 'decide-id' even though it ends in 'e', because we don't say the 'e', so the last sound is 'd'.

So for example:
wanted /want-id/
ended /end-id/
admitted /admit-id/
started /start-id/

For the other two sounds it doesn't matter so much. Just make sure you don't say '-id'! For example, 'stopped' is pronounced 'stopt' and never 'stop-id'.

If the word before 'ed' ends in the sounds 'p', 'f', 's', 'ch', 'sh', 'k', then 'ed' is pronounced 't':
So:
'p' stopped /stop-t/
'f' laughed /laf-t/
's' promised /promis-t/
'ch' watched /watch-t/
'sh' finished /finish-t/
'k' walked /walk-t/

For all other words, 'ed' is pronounced 'd'