The first to do is to sign up as a member of this site (link in the top right corner). All you need to do is supply your email address, setup a name and password and that's it.
Once you are a member you will be able to post blogs on your profile, post photos, videos and create events. You can also comment and participate in discussions. There is usually an + Add Content
link in the top right corner of most pages, or you can use the Quick Add
drop down below your Name in the top right of the screen.
If you would like more specific information on how to use the features of the site, you can find a lot of great information here: Become an active member