Yes, log onto the BA website follow links for Heathrow, and there is a section on which you key in your flight no. and this will tell you which terminal you fly from.
Yes, this is easy to do. You need your PNR which is a 6-digit alphanumeric code - your T/A will give this to you. Then you go to BA.com and on the front page there is an option to enter this and your surname and you can see details of your booking. 24hrs before your flight you can pick your seat assignment.
Yes, go to www.Ba.com then go to my flights type in your booking ref ( if you don't have this contact travel agent) once you have entered your ref all the details will come up, you will not be able to reserve your seat until 24 hrs before flight when you check in on line.
Thanks folks, can I find out which type of aircraft too .
Yes, you can! Log in as it has been described to you, then click on the My Booking tab, then click on where it says the flight number,and it will bring up a little box that gives the aircraft type.