Be a user first
First step in contributing is using - knowing what is missing, what needs to be added.
When you face problems using the software report them to the developers.
Think how the software can be improved and request those features.
Test new versions
Try beta and alpha versions of the software and report bugs.
Build latest code
Download latest code from their version control systems, build and use it.
Fix small bugs
Try to fix small bugs. Even spelling mistakes can be a good start.
Create a user group
If you are in a college, start a user group there. Organize regular meetups and talks on specific areas.
Spread Free Software
Go to new places - schools, colleges and other organizations, and introduce Free Software to them.
Looking for a specific project? See High Priority Projects