Oh you must be so excited, Istanbul is absolutely fabulous. In the hotel etc. its fine to wear shorts and tshirts etc. (though Istanbul can actually be quite chilly that time of year.) But if out and about in the city try to dress conservatively, it is a Muslim country and you might find yourself getting some unwanted attention if you show too much leg/arm etc.
I would recommend visiting the Blue Mosque, its the most beautiful building I've ever been in. You will be expected to cover your head if you do however. Its also well worth going for a wander through the markets, the sights and smells are fabulous.
It depends on who you are flying with as to what your luggage allowance will be. The hold luggage is your big bag that generally has your clothes etc. and your hand luggage is for on the plane with you. Hand luggage must be small enough to put under the sweat in front of you.
Buying things in places like Turkey is very different to doing so in Britain. Take any price that they quote as a starting point, to be wittled down. You can usually haggle to at least half the price. If getting in a taxi make sure to agree the price before you get in to avoid a hefty fee.
If there is a safe in your hotel make sure to put your passport and some money in it, just in case of emergencies. If taking money out with you I would recommend getting a bum pack to put under your clothes (this avoids unwanted pick pocketing which can be a real problem.) And I always keep a note or two in my sock just in case (fingers crossed never had any problems and I travel a lot.)
I would recommend getting some guidebooks (Lonely Planet and the Rough Guide are fab ones and will be able to give you an insight into what to expect as well as good places to visit etc.
If possible (I know it can be daunting first time) try to arrange trips on your own terms. If you book your day trips with your tour operator its likely to cost you a great deal of money.
Finally try not to feel intimidated, the Turkish people are lovely but the men can be a little overbearing at times (e.g. sexual suggestions.) Usually a firm no will do the trick and if you are alone make sure to tell them about a husband (real or imaginary) that you are going to meet. It tends to cool them off a bit.
But most of all have fun! Turkey is one of my favourite places in the world.