As for starting a thread, I'm using the old app as my iPhone 4 is too old to support the new app. It should be similar but could be differerent:
Go to whole forum view and choose which subforum your post best fits into. Make sure you are in that sub forum. In the top right hand corner there is a button and when you click on it a few options appear- choose start new thread.
Idk how different it is on the new app so if anyone else knows they will be able to correct my errors