hiya, like you im fair and have naturally medium blonde hair. if your unsure id have a consultation with your hairdresser, i wouldnt recommend colouring your hair yourself, ive been there and had to wait for it to grow out lol.
your hairdresser will advise you as to which blonde would suit you, if your after a more natural look they may recommend two different shades of blonde which will tone in with your natural colour, or if your after a more wild style they may put chunky thick blonde flashes through your hair.
i have a half-head of highlights with two different colour blondes, it works for me.
if your unsure, maybe start by having your parting highlighted and a few going through your side fringe so you can get used to it and then gradually have more done.
hope this helps :)