A few members seem to puzzle over this, I only ever access the forum via a laptop or desktop but this is how you do it there- phones may be different.
Gor to 'Portal' On the left hand side of the screen is a list of subject areas like 'General Physics, News etc.' Click on the subject area that seems to match your intended thread and you will see a whole bunch of existing threads created in that space.
On the top RHS of the screen you see a notice 'create thread' - click on that and follow the on-screen instructions.
Simples! 
