To register go to the Forum page and click on 'Register' at the top right hand side of the page; you will be asked to enter your e-mail address, a screen name (can be your own or an alias) and a password. When you submit that information you will be asked to enter a word displayed on the screen in distorted letters (this is to stop spammers). As soon as you pass that hurdle an e-mail will be sent to your e-mail address with a highlighted activation link (series of apparently random numbers, letters and symbols). Find that e-mail in your Inbox (or possibly your Junk mailbox), open the e-mail and click on the activation link as instructed in the e-mail. You will now have registered and can visit the Forum to post your own contributions.
When you visit the Forum you will need to 'Log in' in order to post your own topics or comment on others. To do this click on 'Log in' near the top right of the screen and enter the e-mail address and password you used to register. On that screen there is also a tick box next to the words 'Log me on automatically each visit:'. Tick this box so that you should never have to 'Log in' again.
If you don't tick the box you will have to enter your e-mail and password every time you want to post something on the Forum. In this case if you ever forget your password there is also a 'I've forgotten my password' link on the login screen. If you click on this and enter your e-mail address an e-mail with a new randomly generated password will be sent to you (but note you won't be able to change this one!).