Welcome onboard

I've moved your post from the Turkey Forum to the correct topic on the Flights Forum where you'll find all the information you're looking for. Where in Turkey are you flying to? Dalaman, Bodrum, Antalya, Izmir? LGW-ADB-LGW (TOM752/3) are likely to be operated by Boeing 757-200 but I won't know for sure until May. There are 5 seating configurations onboard the 757-200 but they are all essentially the same apart from the position of toilets and an odd seat. You'll find extensive aircraft information on our What Aircraft? guide (link below) including a seat plan. There are no 'best' seats. Seating is personal preference so it's whatever you prefer; window, centre, isle, front, back. All seats offer the same basic facilities apart from extra legroom seating. You may want to avoid seats near to toilets to avoid disturbance.
Darren