Single handle faucet is leaking from the center.

If you remove the handle, you will be looking at the stem that you described. You will see the white plastic piece that surrounds that stem as well. If you go one more step outside of that is a retaining ring that is usually either grey or white that has a few notches out of it. This is a tension ring and when tightened, it will stop the leak. There are two small washers with springs that sit below the ball with the stem that can be replaced if you experiance a leak from the spout and not around the stem as you described. Hope this answers your question. Don't forget to rate the solution when your done! Have a very blessed night.

You need to get a kit to rebuild the faucet which may sound daunting but actually is not. Since you did not state the brand of faucet, which I assume you do not know since you just moved in, you will need to go to a home depot or lowes and find a similar faucet to know what brand you have. Then ask the plumbing expert there how to proceed.