Once you have registered, or if you are returning to a site, you can login to the site. In the menu bar on the forum you will find an image/text that says "Login". If it says "Logout" you are already logged in. This can happen if you've already logged in before and chosen to have the forum remember who are you are. This places a cookie on your system. Once you choose "Login" you will be prompted for your user name and password. If the combination is correct you will enter the forum.
If you are done browsing the forum you may decide to logout. Perhaps you share a workstation with someone else, in this case you would want to log out of the forum upon leaving. When you are logged in you will see "Logout" in the forum menu, select this and the forum will log your user name out.
If your desired user name is available you will either be able to login or you will receive an email with a temporary password - again this depends on what type of registration the administrator has set the forum to.
If you're trying to login to the forum but you forget your password or have lost it, you can retrieve it using the lost password tool. On the login screen, just below the Login button, it says "Forgot password?" click this to be taken to the forgot password form. Enter your user name on this screen and press the Send button. Your password will be sent to the email registered to that user name.