If you're on broadband (e.g. Cable or DSL) then you'll get a flat rate charge every month from your Internet Service Provider. They should be sending you a paper bill in the post/mail unless you have switched to paperless billing, in which case you will need to log on to their website every month to review it. As I said though, it should be a flat rate - the same amount every month.
If you're on dial-up then you need to add together your ISP's charges (accessed as above) with your Telco's charges for dialling the internet number (which will require itemised billing on your phone).
If you're on dial-up then you need to add together your ISP's charges (accessed as above) with your Telco's charges for dialling the internet number (which will require itemised billing on your phone).