if i have apple care on my iphone 8 plus how much would it cost to get my screen fixed?

Assuming that's all you need, $169 for a replacement screen in the US unless you have AppleCare+. With AppleCare+, it's $29.

iPhone Service Pricing - Apple Support

I believe $29 plus tax in the United States. It may be different in other countries.

iPhone Screen Repair & Replacement - Official Apple Support