You need to be sure it's fleas first, get eiter a flea, nit or a fine toothed comb and run it though the fur of your cat, on a damp tissue wipe the comb both sides, any flea dirt will show as little black dots on the tissue, any gernral dirt will smudge and smear on the wet tissue.
Then I'd get yourself onto VetUK (Otherwise I'd be right off to the vet) and order a pack of Spot-on. It's a really good flea treatment you place on the back of the cats neck, and works within 24 hours.
-I will say though if after a week or so your cat still has marks around it's neck and they don't seem to be going down then get yourself to the vet for a check up.