I wouldn't wash it first.
I think the hair color adheres better if it't dirty.
I can't remember why.
My mom used to dye her own hair & wouldn't wash her hair first.
Same when I used to go to the salon....she preferred unwashed hair.
You wash it right after you dye it anyway to get the color out so it will be
clean then.
Don't forget to put something around your shoulders, do it in the bathroom where you can't ruin carpet, put vaseline around your roots so it won't dye your skin, don't leave in longer than it says, it won't help and you don't need the chemicals in any longer than necessary.
Enjoy your new color!