[TIPS?] How to give a shape?

I’d like to see people tips, on how to give a special shape into a box, how to turn it to something, make it be something.
Like, all I know to do is to extrude (And some GTA modding which is mainly just attach and/or replace).
I’d appreciate some tips on how to give a special shape to something.
Like, for a weapon, what modifiers do you have to use, how do you have to do it, etcetra.

For weapons I usually just in mind myself break weapon up into pieces, like barrel, grip etc… And make them separately not from 1 box.

I will be trying that out, thanks.

Any other tips? :o.