I just got an iPhone X and I want to see if I can use my sim card that is in my iPhone 8+ or if I have to get a whole new sim card.
You'll need to get a new SIM. Unfortunately Sprint (and Virgin Mobile as a result) does not allow sim switching between phones. You'll either need to contact customer care and have them ship you one, or visit an Apple store and get a FREE sim card from them. Just ask any Apple Store specialist for a replacement Sprint sim. You can then log into your VMUSA account and swap devices through an easy online process.
I did this when I upgraded to the iPhone X, and it was easy as pie.
Let me know if you need instructions on how to swap your devices using the online process.