A low GPA will hurt you. One F, especially if you re-take it, won't kill you. HOWEVER, be aware that your school calculates your GPA one way, and the programs you apply to may calculate it another way.
For example, if you get an F, retake a class, and then get an A, your school might calculate your GPA with just the A (4.0 value). However, a program you apply to might average them together, giving you a 2.0 value and thus lowering your gpa. It's important to make sure how they calculate GPAs so you know if you truly meet requirements and averages.
Now, an F won't hurt you if you have a lot of A's, however if you have a lot of C's an F can weigh you down. In the same vein, look at your grades as a whole. Is there an upward trend? Perhaps you got of F's and C's early in your college career (I did), but then turned it around getting mostly A's in the last couple of years. While your GPA is damaged, admissions will take note of the trend especially if you have other supporting data to show that you are capable (such as high GRE scores and letters of recommendations).
So in summary, yes, it has a negative affect on applying to graduate school, however the impact is not as deep as you might think. It won't be the sole factor of rejection or acceptance. They won't go, "They failed intro to fitness! He is of lesser stock and must be eliminated from the gene pool!" When you apply to graduate schools, they look at you as a "complete" package: personal statement, UG GPA, GRE, letters of recommendation, relevant volunteer/work experience, and interview.
Early in my college experience, I had to retake a few classes, and I had a lot of C's. I turned things around, however, and I got accepted into many of the masters programs I applied to (all tier 1 universities), and now I'm doing a PhD (at a tier 1 university). My low UG GPA still haunts me in my dreams though...
