Finding the correct station / postcode for National Rail tickets (London to Edinburgh)

Type London and select London (All) if you don't know which London terminus you need. It will then tell you the services you can take.

In the case of Edinburgh, you'll see that trains leave from London Kings Cross (and a few from Euston.)

Then just get to Kings Cross using usual London public transport from Bayswater. Presumably if you're staying in London beforehand you'll already have a travelcard or Oyster which will cover a tube or bus journey to Kings Cross.

It is possible to book a through ticket from Bayswater - and some tourists make the mistake of trying to book from the nearest station to where they're staying - but the first part will be public transport and will already be covered by whatever ticket/pay method you're using for London's transport.

It just means "enter station name or postcode". If you enter a postcode it will do you routing to the nearest bus stop.
Most of the time the station name is the same as the town name. As Jane says, just enter "London" and it will ofer "London (All)" as an option. Enter "Edinburgh" and it will offer "Edinburgh Waverley" as the main station.

any particular information I need to know on certain train routes

If you just enter start and end stations, it will find the quickest route.

Thank you both for the information! Is nationalrail.co.uk the best way to get tickets?
Here are two options:
(This first one has one change EDB to HYM but both are the same price. Any thoughts?)
11:30 London Kings Cross [KGX]
Platform 0
Haymarket [HYM]
Platform 4
16:30 5h 00m

12:00 London Kings Cross [KGX]
Platform 8
Haymarket [HYM]
Platform 4
16:37 4h 37m

The main station in Edinburgh is Edinburgh Waverley. This is the one you want, not Haymarket.
You should not have to change. There are trains every half hour from London Kings Cross to Edinburgh Waverley, taking just over 4 hours.

Edinburgh Haymarket is a station slightly outside central Edinburgh. Some trains from London continue to Haymarket (and on to other destinations), some terminate at Edinburgh Waverley.

National Rail should be your first stop for times and tickets. It will automatically transfer you to the correct website for the company operating the trains - in this case Virgin East Coast.

Thanks again! This information really helped! I found a couple different prices for the same route and ended up using Virgin as they had a cheaper price.

If you paid less, it is almost certainly an "Advance" ticket which has certain restrictions on changes (expensive, difficult and limited) and refunds - there aren't any. You need to be sure, much like an airline ticket, that you travel on exactly the time and date train on the ticket or you will be holding a worthless bookmark.

Ok got it, thx!

Excellent information. Thank you to the original poster and to all who responded. Very helpful.

Yes, thanks everyone for the great info!