First of all, make sure you're using every possible resource available to you to get the best grade you can! You can find some good videos on YouTube. I found Khan Academy to be extremely helpful for visuals and easy to understand explanations. Take advantage of tutoring, if your school offers it. Go to office hours and ask the prof to review things you don't understand. Do practice problems from your textbook or ones you find online. Is there anyone in your class who seems to just get it? If so, befriend them and ask for help!
If you end up with a C, it's not necessarily the end of the world. If you get anything below a C, I would definitely retake. Personally, I think whether to retake a C is somewhat negotiable. It depends on the quality of the rest of your app. (Is your GPA otherwise strong? Do you have C's in any other prereqs? I don't think one C won't kill you but several certainly can. If you have other academic weak spots, a retake might be more important for you.)
I've never heard of adcoms looking at any one grade first and I think that's pretty unlikely. It's more about the whole package than it is about any one course.