I could type a snarky, condescending "RTFM" and "Search the forums" post here, as per the usual suspects, but instead, I will take a few minutes and assist you.
Random car parts that are either bought and not immediately installed or gifted to you, will appear in the "Items" list when you open the little suitcase from the GT Life menu. They can then be installed into the current car that you are driving from there. Horns work the same way.
To change helmets/suits on your A-spec driver (also the one that people see in your profile and online), click on your current profile picture in the top left corner of the GT Life screen and it will open a window with your driver slowly rotating while wearing the current helmet/suit that he is equipped with.
You can change B-spec driver's helmets/suits from the "Drivers" menu off of the GT Life screen.
