How can I write a Crontab that will run my /home/username/test.sh
script every 2 hours?
Just do:
0 */2 * * * /home/username/test.sh
The 0 at the beginning means to run at the 0th minute. (If it were an *, the script would run every minute during every second hour.)
Don't forget, you can check syslog to see if it ever actually ran!