speak to you provider and ask them to send you a replacement sim, you will keep the same number
or you can buy a sim, but you will have a new number. If you want to keep the old nunber you can contact your provider and ask them for a PAC code, you then give this to the new provider and they port the number over.
Get the new sim and once it is in the phone call your mobile provider and they will be able to change the new sim to your old number, may take a day or two though. I've had to do it more than once!