hi sophie,
your best bet is to always moisturise your face after washing, let that sink in a bit before dabbing with a towel to get the excess off before doing your make up.
if you have reasonably good skin, i think youd only need a light makeup to give you a nice even coverage.
nip to boots and get yourself a little pot of their own brand 'natural' loose transclucent powder. i use that, its only about £2.50, and its great.
just lightly dab, with one of those small flat white round powder puff things (you can get a pack of 3 for about £1) over areas of your face after youve applied your foundation/concealer etc, to kind of 'set' your finished look.
being transclucent it does what you want it to do without really showing if you know what i mean.
if you go to a bigger boots shop, they will be able to advise and help you with what foundation would be best for your skin type and the look youre after.
they will even give you a little bit of makeup into a tester pot for you to take away to try at home.
the body shop will also give you help and samples too.
youre still only young, so try and keep your make up looking nice and fresh and light, you dont want anything heavy :o)