Can I jailbreak a brand new iPhone 6 plus?

I think it's running iOS 8.

It will be iOS 8 since that's what it launched with on Friday & no you can't since there's no jailbreak available.

Right now there's no jailbreak available. I wouldn't jailbreak it just in case anything happens to it if one does come out. Remember, you are ditching your apple care if you jailbreak. So if you mess up your iPhone, you can't do anything about it. You must deal with it yourself.