Look at the forum structure and go to the GENERAL/TALK area for the model year Mustang you want to talk about.
Click the POST NEW THREAD button, give your thread a title, then ask your question or post your information.
If it's a purely technical question, you would put it in the technical forum. If it's an opinion question, you would put it into the general/talk section.
If you get it wrong... one of our moderators will put it in the right place when they find it.