The relevant governments will be more than happy to help you with this. For example this link. You can get them on the phone too if you're having trouble figuring out what to put on an invoice the first time for that client. If in doubt, ask.
You generally do not charge tax for clients outside of Canada- they are "zero rated" not exempt. That allows you to use the ITCs for associated costs.
You will generally charge the appropriate rate of federal tax (HST or GST) for provinces outside your own. You never charge provincial tax for provinces other than your own (only HST where applicable).
Exceptions may exist for clients who have multiple locations (or if you had multiple offices in different jurisdictions) and it can get quite complex - for example with drop-shipping physical goods and such like, but it sounds like your situation is simple.
Be careful if you're doing work for a client who appears to attract a lower rate of tax because of their location, but you know they have a physical presence in a location that would attract a higher rate of tax.