I echo Jen and Semiferal - NEVER use a flea collar.
Over the counter medications for fleas do not work, are a total waste of your money, and can be dangerous.
The very best thing for you to do is take kitty to the vet, have him prescribe flea medication according to her age and weight and follow his/her advice.
If you have already bought a flea collar, cut it up and put pieces in your vacuum. Vacuum thoroughly every day and empty the container outside each day.
Wash any linens where your kitties have been and also vacuum your furniture.
The medication from the vet will probably be the kind that lasts for one month, i.e., it gets into the cat's system and any fleas that bite the kitty will die in a couple hours. Also use a flea comb and drop any live fleas into a container of water and dawn dishwashing liquid - this will drown the fleas.