To post a new thread using the web version of the site (not a mobile app)
You go to whatever subsection you want to post a thread in. Here I am using dollars and cents as an example. Click the "New Thread" button, (see in top left of pic)
Now fill in the title box and post body box. Scroll down and click on the "submit new thread" box. Shown in second pic.
However, if you want to select more options and may add an attachment or pic, scroll down further to do this and then click on the "submit new thread" button below these options. You can also preview your post at either stage too, by clicking on the preview button, but you still have to submit it before it shows up in the forum.