First you have to become a registered user. Just click on the Register option in the Menu area. Fill out the various fields and you're ready.
Once you're registered, if you have a new topic that you want to add or start, first figure out which discussion Board it belongs under, Winter Trips, Socials, etc. Click on the Board and then click on "New Topic". Just type in the subject for your topic and your question or comment, and click Submit.
If you want to reply to an existing topic, just go to that topic and click on "Post Reply". Type in your reply and click Submit.