Good on you. This is something you can break into small chunks to make it manageable.
I would buy a ticket in advance - if you download the trainline app, you can buy your ticket and it will be on your phone ready to use.
When you get to the station (think it's either Kings X or St Pancras for the St Albans train) you'll see a massive departure board with all the trains on it. Look out for your train, and it will tell you which platform to go to.
From there, you will probably have to go through a barrier. The ticket on your phone will have a barcode on it which you'll need to bip on the barrier to get through. There are always staff on hand to help out, so don't worry.
Follow the signs for your platform and your train will be there. There will be screens on the platform showing the train's route if you want to double check.
If your ticket has a seat reservation it will tell you which carriage it's in (will be a letter). Find that carriage and then your seat number, and you're off. If you don't have a seat reservation, just look at the little displays above the seats which will tell you if they're reserved or free. You can sit in any free seat.
Once the train is moving, all the stations will be announced. Just before you get to St A, it's worth getting yourself ready (coat on etc), but you'll have time to get yourself off when it stops. You may have to bip out at the barrier in St A the same as on the way there.
Going on your first journey out of London is good because it's busy, so there are lots of staff, and also because you leave from a terminus so you don't have the uncertainty of waiting on a platform for a train and then wondering if it's the right one.
Hope that makes sense? Good luck. You can do it.