Get it done at the place you trust.

What color do you want? I see your avatar and sig as 'black' and that's simple enough to do.

I also read the word 'purple' here and there. To do purple you have to lighten your hair (if it's darker than purple) and then dye it purple. It should last a few days but should wash out (or become diluted) in a few days because flat color (ie not black) hair color tends to do that.