public class AuthCallout {

   public void basicAuthCallout(){
     HttpRequest req = new HttpRequest();
     req.setEndpoint('http://www.yahoo.com');
     req.setMethod('GET');

     // Specify the required user name and password to access the endpoint
     // As well as the header and header information

     String username = 'myname';
     String password = 'mypwd';

     Blob headerValue = Blob.valueOf(username + ':' + password);
     String authorizationHeader = 'BASIC ' +
     EncodingUtil.base64Encode(headerValue);
     req.setHeader('Authorization', authorizationHeader);

     // Create a new http object to send the request object
     // A response object is generated as a result of the request

     Http http = new Http();
     HTTPResponse res = http.send(req);
     System.debug(res.getBody());
   }
}