How to harden a steel rod?

its like a steel rod but flat i foget what it is called i got it in a mettle store it is like 2 inches by .5 inches and 3-4 feet longish and i have ben trying to turn itv in to a ninga sword i sharpened it with a grinding stone i made a handel and everything but it is to bendey and if you hit it on something like a tree it shakes and distorts how do i make it harder and stronger ?