It's easy, just use the functions that TFS has already made :)
function onUse(cid, item, fromPosition, itemEx, toPosition)
local str = 2000
doPlayerSetStorageValue(cid, str, getPlayerStorageValue(cid, str) +1) -- See what I did here
doPlayerSendTextMessage(cid, 19, "The storage ".. str .."'s...