welcome to TWC;
Im in the exact same shoes you are, trying my first real go at an eastern faction as the turks. Im on turn 33, so far my armies have all been very impromptu due to the chaos of this

starting position. But I think once I get going im going to try for:
4 Horse Archers
2 Heaviest cavalry I can get
7 Infantry
4 Archers
2 Javalins
Maybe even considering swaping out an archer and an infantry for two more horse archers, those horse archers are

ing. As far as seiges are concerned I have no clue, I usually wait for the enemy to sally; if at all possible try to draw the enemy out on the campaign map, thats what ive done in the past. Anyways, let me know what you try and how it works out.
EDIT: and never forget that alt-clicking with your horse archers basicly turns them into watered down light cavalry, great for running down routers; not so hot at melee fighting anything other than peasents.