Are you suppose to wash the t-shirt before you tie-dye it?
i have a brand new white t-shirt that i am going to tie-dye.
i don't have insrtuctions, so i don't know what to do.

do i wash it first in the washing machine with soap.
or nothing.

Yes. Always wash clothing before dyeing it. Otherwise, you may get pale splotches where invisible finishes from the manufacturing process remain to repel the dye.

Use hot water and detergent, and run the washing machine as you usually do for laundry. If you have extra soda ash or washing soda, add a little when you do the prewashing, to make the detergent more effective.

Don't ever try to dye a stain-resistant t-shirt. The results are disappointing. Also, dye only 100% natural fibers, such as cotton. If you dye 50% cotton/50% polyester, you will get pale colors, and if you dye 100% polyester, the dye will wash out almost completely.