I had my first ultrasound on january 15th, 2013 and it said I was 18 weeks pregnant, what is the most likely date I conceived?

Subtract 2 weeks. When "calculating" this, we assume a 28 day cycle with ovulation on day 14. To get approximate conception date, subtract 2 weeks from your current gestational age. That will then be how many weeks ago you conceived. So in your case, your date of conception is approximately 16 weeks prior. This works better in first trimester as dating gets less accurate further along.
Take out the . Calender and subtract 16 weeks from january 15th. That day, depending on the accuracy of the scan, is within about 48 hours of your conception.