There are some different functions,in simple sentences, use は because this is the topic of the sentence, eg: Heは is going...tomorrow.
In clauses, が is used as the grammatical subject of the clause, eg:
'John knows that Tom is going to..' = 'Johnは Tomがis goingto... knows'
For contrast, は is used, eg:
'We don't sent Christmas cards, we send New Year cards' = 'Christmas cardsは don't send, New Year cardsはsend'
There are other uses too, but hopefully this will help. :)