Are you expexting your af to come on the 20th of january????
If so, then you can take a test the earliest on jan.16th - 4 days before you're expected af. Buy a "first response" test, they work the best for early testing. I would buy a pack of 2 just in case the first test comes out a bfn, and you still do not get your af on the 20th, you could wait a couple of days and then test again.
Good luck!!!!