Visit YaBB! YaBB 2 Quick Installation Guide - Setup
Now it's time to run the Setup utility provided in the zip package. In your browsers address line type:

http://www.mysite.com/cgi-bin/yabb2/Setup.pl

You must type in the system password ('admin' as default).
The Setup utility will now search for the paths to your forum files, and show them in a form to let you check/correct the setup.

After setup you should be ready to login and test your new forum.

After you have successfully run your YaBB board from the web, you need to try logging in. The default username is "admin" and default password is "admin". They will always be so when you install a new YaBB board. Login with that information and you should see an "Admin" icon in the menu of the YaBB board. Clicking it will take you to the "Admin Center" where you can see all sorts of nifty things to do.

Test everything (that you know exists), such as posting, registering a new member, modifying message.. etc. Then after that, go to the admin center to create some new boards.