If you want to take the TOEFL IBT, there's no shortage of dates to choose from. It's offered 30 to 40 times a year at thousands of locations around the world. The PBT, however, is only offered 6 times a year.
To find a location, the next available test dates and the fees, go to this ETS page. To quickly look for just dates and locations, use this ETS page instead.
You need to have 1 or more dates and locations picked out before you can register. You should pick a few alternate dates incase your first choice has already filled up. The sooner you register, the more likely you'll get your first choice. You can register a few months before the test date.
Even though there's lots of dates, don't wait too long before registering. This is especially true if you need to register by mail. If you decide to mail in the form, ETS must receive it 4 weeks before the earliest test date you choose. So if you can, do online registration. It's the quickest method and available all the time. You must have a credit card though.
ETS stops registration 7 days before a test date. They do allow late registration up to 3 days beforehand, but it will cost you an extra $35. If you need to reschedule your test date you can, but it will cost an extra $60. You also have to contact ETS at least 3 days before your test date. The 3 day limit applies as well if you won't be able to take the test. Try not to cancel because you only get half of the registration fee back.
When you have registered successfully, you will get a registration confirmation letter which includes your registration number. You can use this number to login at the ETS website to check if the date or location of your test gets changed. You can also use it later on to see your score.
So the other important date is when do you get your score. If you took the IBT test, you will be able to see your score online 2 weeks after your test date. ETS also mails out your score report to you and the institutions you selected after 2 weeks. ETS is in New Jersey, USA so how long your score report takes to reach you will depend on where you live. If you took the PBT test, your score report is mailed out ~5 weeks after the test date.