I don't know what you mean by "url of db2". Like Andy said, you need to login as the DB2 instance owner id.
Check if you have this file: /var/db2/global.reg
If you do, you can view it using "strings /var/db2/global.reg"
This file will give you the instance name - for example db2inst1. If this file is empty or doesn't exist, then most likely the instance was not created. In this case, login as root and use db2icrt command to create it. See this URL for info on how to create an instance:
IBM DB2 9.7 for Linux, UNIX and Windows Information Center
Then, switch to the instance using "su - <instance name>" or login as <instance name> directly and execute db2start