im 15 and doing well at school. im studying maths,english, science,history,art,ict and spanish. im doing well in these subjects but dont know how much they will help in attaining my favoured career. should i be doing extra lessons, after school sessions? i know i wont be able to become one just like that but i want to know if i am just wasting alot of my time.

Try getting into an IT subject, and computer programming. If there's some sort of IT club on your school, that could be favourable.
Many great developers have modded other people's games before they became developers, have you tried that?

Yup, get into programming and you should be all set. Learn scripting and what not.

Since you are only 15, it is hard to tell.
I'd say you're going in the right direction by taking math in school.
But it's still "school". I think your ability to become a game-dev will be more dependent on what you do after school. Like college/university.

by modding do you mean with things like the geck or other rather 'expensive' software?

Start off with using tools like GECK and the CK, then try your hand at Scripting using C++ or whatever tool you prefer. It's hard to get into, but it's probably a vital step towards your dream.
Bethesda games are quite easy to mod, so try other games later.

how do i get into programming exactly?

do i need fallout to actually use the geck?

I'm sure there must be a computing course in your school. If not, perhaps in high school. There are different kinds of developers that specialize in different things, so for now just get a good grip of the basics. Starting out with stuff like GECK is good.

Books, courses, online help.

Online would be fine to get you started. Search for stuff like "c++ hello world" or something.
Books would take you to the next level. Teach you the basics of the language.
Courses and classes if you are really into it. But then again, good change you will get programming classes if you start an education towards game development.

School subjects, modding guides, generally showing enthusiasm and interest in the subject can get you good grades.
Like you, I'm 15 too, so I do what I can to go towards my dreams. I would love working with history or social science (is that a term?), so I study those subjects hard at school and try to sponge up as much info as I can.