The tickets will start to be available from the Fifa website from 20 August. If you click here you can see the options. In this first phase they will be allocated by random selection, and from November they will be available on a first come, first served basis.
To be available for a chance of getting a ticket as soon as they go on sale, you need to apply for tickets. If you have an account with Fifa you can log in now; if not, you should register here. You can then fill in a form (all details are given on the same page) and you will be told what steps to follow next.
At present the cheapest ticket prices for fans outside Brazil is £288, but further changes could still be made.
By the way, in previous years there were some fake emails claiming to come from organizations linked to Fifa, and offering better prices etc. Please be very careful with these - Fifa is the only organization you should trust in this case.