Obtaining a phone line and setting up broad band is done at the post office and at the Turktelecom office which is located at the top of the stairs on the left hand side of the PTT building as you stand in front of the PTT.
There is a set up charge for broadband but it is small. You will need a modem
( there are several computer shops which have them and will install them for you.) Turk Telecom do not do this, they just deal with the line. You can also get a wireless modem, if you are using a laptop, a bit more expensive but worth the extra for the flexibility to use your computer anywhere in the house.
We use the computer shop next to the Nur pastanesi, located on the road up to the market, where the IS bank is. They speak English and are very helpful.
To pay, if you have a Turkish bank account, you can set up a standing order or direct debit. This is the easiest way, otherwise you or somebody on your behalf must go tp the PTT each month to pay the bill but beware, broadband is cut off if you are more than 5 days late with your payment.