Amanda,
As a former backpacker to London, here is my suggestion...
Buy a day off peak (after 9:30am) travelcard for £10. This includes your travel on Southern train into Victoria Station. You are on holiday, so no rush to get there and you can enjoy the passing scenery if it is still daytime. This card can also be used for the tubes and buses which will save you money.
Gatwick Express and Southern (as do First Capitol Connect and South Eastern) arrive at the same station.
So here is what you need to do once you arrive!
1. Use the loo, they are free
2. Withdraw money if you haven't exchanged money in advance. There are loads of cash machines and exchange desks all over.
3. Find out what terminal you are in- North or South
4. The train station is in the South terminal which is just an connection away from the North... signs are EVERYWHERE... and people speak English
5. Before you go to the platforms downstairs, buy your tickets... there are machines and the line is tucked away in a corner... I always forget to get my ticket and end up having to walk back up.
6. Buy the day travelcard to London.
7. You can also buy other travelcards for the days you will be in London, up to 7 days in advance... so if you know you will be around for the weekend you can get a 2-3 day card (no cost savings, but all on one card)
Good luck! You will be FINE and a pro by the end of it!