How much does one letter grade affect my college app?

I ask because I have had ups and downs in high school. I have always been one of the smarter kids in my grade (and modest as well) but I am a bad student. Always have been. I do little homework, and don't pay much attention, but still get A's and the occasional B. Unfortunately for me, math doesn't work liket that. I had a B in first semester Junior year (the rest were A's) and this semester I have a C. It lowers my GPA I know, but how much will that C affect my applications? For the record I am thinking about applying to Emory University, which is a top 20 school, or maybe Wesleyan. So not easy colleges to get into. My SAT score is definitely enough to get me in, and I can write good essays/get good recommendations. I had a C in math last year as well for one of the semesters.