Hi,
Suppose you want to start a thread in millitary aviation---that is on the main page---you click on it---it will open a new page with all the posts---then you scroll down 1/4 th of the page---where you see the threads---look at the left side---it will say---THREADS IN FORUM : MILLITARY AVIATION---right on top of the word THREAD, you will see 'new thread'-----click on that.
To start a new thread, you have to have a minimum number of posts---check it out if it works for you.