Can someone pls tell me how to troubleshoot DNS errors?

Can someone pls tell me how to troubleshoot DNS errors?

When i try to connect to the internet, i get a message saying DN could not communicate with host. Sometimes the message is security settings are blocking netwok connection, the message states windows firewall query user. Other times when i check the status of my connection i am receiveing alot of information but there is still an X from the router to the internet. this problem is driving me crazy. I have DLS connection in which i connect wireless thru a linksys router ,my pc uses windows vista....Can someone please assist. I tried everything and nothign works.