how to set the user credentials for accessing an webservice

Question

hi,

i have an webservive written where any user hit the url can acces the webservice.now i need to set username and password for the webservice so that before accessing the webservice they need to provide the user credentials only valid user would be allowed to access the webservice

how can i achive this functionality. is there any setting tat i need to do

any help would be great

thank you