I'm jealous of my best friend's new friend.
Why are you jealous of your friend's new friend?
I want my best friend to myself.
It's normal for people to have more than one friend. Instead of being jealous you should be happy that your best friend has found another friend.
I just don't like the new friend.
It's pretty natural to be jealous, but it's not nice  to react that way. Maybe you can try to arrange to see your friend alone sometimes so that you can keep your friendship strong even as she makes new friends.