Below will be some items just to help everyone get started:
Your profile - Some cool things about this board is you can set your display name to something totally different than your actual login. Like my name shows up as AL Gator when I post something, but my login could be humptydumpty if I wanted it to be.
To edit your profile simply hit the button at the top of the board that says 'Profile' and then click the link that says 'Modify profile' at the Profile screen.
Instant Messages (PM's) - In the 'Info Center' below the main board, in the bottom right hand corner it will inform you if you have any instant messages. Click the number of messages you have and it will take you to your inbox.
Avatars - This works just like most other messages boards, but you can specify what size your image is. If your avatar is 100x100 and you don't specify that it is 100x100 it will default to a different size and it won't look as good.
Please note that ProBoards does not directly store images. If you want to use an image for a post or avatar, you will need to upload it to an image site (there are several free ones around) and link to it.