Hi Mavis,
Welcome to SaleHoo!
Are you using an ecommerce package (e.g. Zencart, Shopify etc) or have you had a website built from scratch?
If you are using an ecommerce service, then you should be able to find instructions in the Help section pretty easily for this. (Or I can look if you let me know what one you are using).
If you have had your own website built, then things get a bit more tricky and you'll probably have to ask your developer.