Go and have fun
Ok, you are going to be doing some extensive study for the next year or so. Now's the time to learn by enjoying yourself. You shouldn't have to spend much money because your school should supply the texts.
Texts. Kermode is a good one.
Get an aviation term dictionary.
CAP 371 isn't bad to get a grip on the phrases.
Aircraft Performance Theory by Swatton is a good text and covers PoF and Performance.
The Jet Engine by Rolls Royce is a beauty but pricey.
Aircraft Instruments and Systems by Pallet is great but pricey.
Now, if you can, get yourself to some aviation museums and actually see what you are going to be studying. A picture is worth a thousand words and a 3D cut away model is worth even more. One such museum is at Coventry and has lots of cut away turbine engines + piston engines.
Wander around the various aircraft and see if you can identify different sorts of leading edges, winglets, landing gear types and so forth. It makes much more sense seeing them on the various aircraft than reading words in a book. Plus, it's a great day out.
Start watching the weather on the news with interest. See how the fronts move and note the weather associated with it. Surf the Met Office web site for more info.
Maths per se isn't too hard. Just know how to transpose formulas and brush up on your trigonometry. (cos, sin, tan etc)
You'll be using the wonderful CRP 5 for your nav questions. Get a hold of one and start practising how to use it accurately.