wheel offset, etc

if i want to go to a 15x7' wheel, what do i need for offset, i've read 39.9mm, but is theis positive/negative and what is the difference. i want something else to choose from except wires, panasports, konig and stock.
thanks in advance. they will be going on a 1973 tr6 stock brakes, drums, etc.
:popcorn: