Welcome to Flyertalk stephaniews, welcome to the BA board, and I can see this is going to be a big thing for you. But the good news is that you've come to the right place, and there are some great people in this forum who will assist you. I very much hope that despite the daunting experience that it could be for some travellers that you enjoy it all. This forum is here to put your mind at rest, so feel free to keep asking questions.

I endorse the answers already given (particularly to use thebasource.com and avoid seatguru, it's just an advertising dragnet).

Two suggestions from me in addition to the points already made: make sure you have plenty of time before travel so you can gently get through the system without feeling rushed. I deliberately do the opposite, otherwise I'd never see the real world, but for a first time traveller I think you should aim to be at LHR a good 2 hours before departure and enjoy the lounge.

Also, perhaps bring with you some chocolate/fruit/boiled sweets/biscuits in little plastic bags in your hand luggage. Also bring an EMPTY 1 litre plastic bottle, then fill it with water once airside in the lounge or at the water fountains in T5. There is plenty, plenty to eat and drink on the service, and you can always use your call bell to get a cup of tea, but if you are a bit delayed taking off, or wake up mid flight, it's very handly to be able to reach over to your own supplies.