Convert or open Oracle 7.3.x database in Oracle 11
You should open your 7.3 database with Oracle server version 7.3. Than you need to export data using legacy exp command and import generated file into Oracle 11g using imp. 
That would be safest path. 

If I remember correctly you can not migrate database directly from 7.3 to 11g version.   

EDIT: 
As mentioned in comment, there are more ways to to get to 11g version. Here is upgrade guide http://docs.oracle.com/cd/E11882_01/server.112/e23633/toc.htm