Go to the forum/section you want to post a thread in through the "forum" button at the bottom of the app. The "forum" button is in between the "latest" and "search" buttons.
After you are in the section you want to post in such as "Chi Chat" in the top right hand corner you will see a button in the top right hand corner that looks like a forward button from a e-mail but it is backwards (little arrow pointing to the right.) If you click on that button it will bring up several options. "New Topic" is the second option under that button. Click on the "New Topic" button and then post as per usual.
I hope that helps