If my LMP was Dec 12, 2011, when did I conceive? And how far am I? I have gotten confused with the subtraction and addition of the two weeks. Do I start from counting from the 2 weeks after my LMP?

There is no way to know for certain, unless you had specific forms of medical assistance to become pregnant. About two weeks after the first day of your last period is the guide that is used for a likely conception date, but this can be off by days either side.

When determining the number of gestational weeks, it starts with your LMP. When determining due date, rough guide is to count back three months from LMP and add one week.

Again, these are only guides and certainly not exact. Have you had an ultrasound? That is another way to get an idea of about how far along you are.