How to decide what ur website or domain & blog name

check the availability on whois.com or godaddy.com

but the domain name should be specific to the keywords being used to search that website.
eg: u want a website from wher u can sell the cars like carwale.com
So the name of the domain name can be:
www.carwale.com

 u want a website from wher u can help about education like edutraffic.com 
because people will search :
education, recriument, placements, job portal google.. so think accordingly.