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