primo

There might be debate over whether vocal cords are considered an instrument but I didn't know where else to post this..
Anyway, somebody was telling me if you sing a lot and aren't doing the techniques spot on that you can damage your vocal chords.. Is that true?
I want to improve my voice; I went to 3 singing lessons but I can't afford the money to continue it at least for now, so i've pretty much just been singing for roughly 30 minutes a couple of nights a week.
I can sing in tune/pitch (though my voice would be considered weak), and try to breath in the right spots with the right posture (sticking my chest/diaphragm out) and I think I know if I can't reach a note and am straining. Does anybody have any tips on singing without a teacher?