OK, here's my experience, gained (often very painfully) over numerous Turkish campaigns.
Firstly, avoid them if you can, they're pretty slow and most of your starting troops are quite quick. The Byz can't build many of them, so concentrate on walloping their Kataphractoi and Trebizond archers instead.
If you have to tangle with them, use horse archer (or better yet, Turcoman horse) units (preferably working together as a pair, see the MTW Unit Guide in the Guides section) to wear them down until they're exhausted and then finish them off in combat.
If they're charging your line, front them with spears and hit them from behind with ghazis, whose armour piercing bonus and high morale will see them through.
Expect heavy casualties, but remember that your troops are cheap and replaceable, Varangian guards are expensive and are only really going to get recruited in Constantinople due to their high tech requirements.
If you can hire any crossbows or javelins at all as mercenaries, do so and keep them exclusively for use against Kataphracts and Varangians.
If you want to make the Turks work long enough to get the amazing Janissaries into action (at which point everyone else is basically doomed), practice skirmishing with horse archers. Practice a lot.
Good luck!