I am a poor/broke 20-something, I make minimum wage (or a few dollars above it.)

I've been trying to build up credit, in case I need it someday - the likely event my aging vehicle explodes, for example.

My current strategy is to use a credit card and simply pay it off on time. However, my question is thus: if I pay with my credit card, then pay it off immediately, am I actually building up credit? Why or why not?

What is the most efficient way to build up money with a card? What else can I do?