Hmm..  There aren't any maps that I know of which let you pick up
weapons (although there's probably some 3rd-party ones), I assume you
mean weapons that other people have dropped when they die.

You have to buy them with the money you earn from winning rounds (or
money saved after losing).   Make sure you've got a key bound to "+buy"
and press that at the start of a round, at your spawn point, to bring up
the menu to buy weapons & equipment.