how to give user server name instead of ip when connecting through SSH?
You have a few options.
If you need access from only a few locations, or a single location, update /etc/hosts
sudo nano /etc/hosts
Add an entry for your server
66.90.115.178 myubuntu
Second, purchase or find a free DNS service, http://www.noip.com/free/ or similar.
Third, ssh uses your current user name by default, so if you have the same user name on both client and server, just ssh server_name
ssh your_server
You only need to specify a user name if the names are different or you want a new user
ssh new_user@your_server