How can I log hours that I have contributed to the project?
You can log your contributions by either:


signing up for an account at lab.civicrm.org which is where most of the work goes on and many community discussions happen; then "joining" a project; then recording your time in a comment using a special code.  

or
signing up for an account on https://civicrm.org and then using a special work log import form.


This process, as well as what is and isn't considered a "contribution" is set out in detail in a wiki page in the gitlab project called Community Engagement. The relevant page is:

https://lab.civicrm.org/community/community-engagement/wikis/Tracking-Contributions