I had sex on December 28, 2012. I still haven't had my period. My last period was December 9-15, 2012. I don't know if or when I should take a pregnancy test! Help!

When according to your cycle is the Jan period due ?

If y miss the Jan period ( appears you may have depending on how many day cycle) you wait 14 days past the due date of the missed period,

Then test with first urine of the morning.
Rough estimate I would wait 3 or 4 more days