To start a new topic, enter the part of the forum you'd like the topic to be in (be it in the Lists section, the Bright & Cheery area, or wherever.)
Find the New Topic button, which will on the right-hand side towards the top of the page (or if you want to start a new topic in an area where you're reading a post, the button will be next to the Reply button, both at the top and the bottom of the thread.
Click on New Topic. Under Title of Topic, type in whatever you want the topic name to be. ie. This topic is called "Hi to all I'm new here..." There's no need to put anything in the Description -- I only add that part on bundle deals so that people can see what games are in the bundle without opening the thread.
Type whatever you like in the box, then scroll down until you see the Send button.
Voilà, you've started a new topic! You can edit your posts and topics at any time by clicking on the Edit button at the bottom of your post(s).
If you need more help finding the game topics, just let me know!