Can i tie dye this or will the black parts be ruined. this is my favorite shirt and i wanna make the white rainbow but don't know how.

I wouldn't recommend dying it... although black is a dark color so it may not affect it.

You can tie dye it but bleaching it first would be a better idea then tie dying it. But, for the rainbow part, if you want a spiral here are the steps and go on YouTube if you want further and more detailed instructions.
Step 1: twist where you want the spiral. So you lay a 100% cotton t-shirt out on a flat, smooth surface. Place a fork in the location you want as the center of the spiral, and twirl fork to create the spiral. Caution: Make sure that any forks used around tie-dye chemicals are not used for food. Carefully work the loose outer pieces into the establish folds to complete the outer spiral.
Step 2: Tie the spiral using rubber bands to make sure it's safe.
Step 3: Prepare all the dyes and soda ash fixer.
Step 4:Soak the t-shirt in soda ash fixer.Caution: Wear gloves when working with the soda ash solution. Completely submerge your tied t-shirt in the soda ash solution. You will need to squeeze the bubbles out of the tightly wound spiral tie-dye designs to make sure that the soda ash penetrated through to all of the fabric. Allow the t-shirt to soak for 10-20 minutes. When finished, squeeze the excess soda ash solution out of the t-shirt.
Step 6: Select the colors you want to use. While the shirt in soaking in the soda ash, select the color(s) you wish to use. The rainbow spiral example was made using fuchsia, orange, yellow, green, turquoise, and purple.
Step 7: (Last step :D) Dye the shirt. Caution: Wear gloves and an apron or old clothes when working with the dyes. Note: When dying spiral tie-dye patterns, you will need to apply dye down into the folds of the fabric. If the dye is only applied to the surface of the disc, it won’t penetrate very well and large areas of the shirt will remain white.
And you should be done, bye.

Because it's sugar that us the answer