i'm new and i would like to start making my own content so How do i Make a hat or helmet? if its possible in sims 2

Yes, it's possible - but it's time consuming, can be difficult, and requires a keen mind and attention to detail. As you missed the big purple "If you are about to post about sims, then you are posting in the wrong place!" that displayed before you posted here, I have a feeling that it may be a bit beyond your skills at this point. Meshing is hard. Reading is easy.