Scroll down to see the punctuation guide. You might find it useful to have this page open when you're writing, or when you're correcting work that your teacher has already marked. In my classes, when I correct your work, if I mark something with a (P), that means there's a punctuation mistake. For example:
We stopped in hanoi for the weekend
('Hanoi' is the name of a city, so it needs a capital letter at the beginning.)
The Prime Minister who has recently returned from holiday spent the day in emergency meetings,
(the part of the sentence from 'who...' to '...holiday' is a non-defining relative clause (that gives extra-information about the subject), so we need to put commas around it.