To create a forum thread, you can:
Method 1
Go to the home page. At the top of the page, you will see your avatar or image. It will look something like this (without the red numbers)
1 - Select a group that you would like to create the thread in. The more groups that you belong to, the more group options will be in this list.
2 - Give your forum thread a title.
3 - Write your message.
Once all of these sections have been filled in, the "Post" button will become active and you can submit your post.
Method 2
Go to the group that you want to post in and type in the title where indicated in the screenshot below. Then type in your post and press the "Post" button.
You must be a member of that group before you can post to it.
Note
You must be a member of a group before you can post to it. If you try to create a thread or a post in a group that you do not belong to, the system will ask you to join the group first.