I think a C (or even a couple of them) will not kill your chances but you want to avoid a string of them, especially towards the end of your undergraduate tenure. If you continue to have trouble in a subject it is important to get the appropriate instruction (from the course TAs, the professor's office hours, other tutoring that may be available) to help you understand why you are having difficulty and what you can do to overcome that and master the material. A B is will look much better than a C but sometimes people get Cs...
As far as the MCAT: There are 3 sections, each graded independently, and each with a maximum possible score of 15. The highest score one can achieve, then, is 45. In practice, however, the highest anyone has achieved is a 43. I think this is because it is just very hard for "lightning to strike" on all 3 sections. Individuals get 15 in a particular section, but it is very difficult to achieve this score in each section. You can look up the percentiles and other sundry MCAT information at
www.aamc.org/mcat. The scoring is based on a holistic scale and the distribution follows a bell curve. The average in each section is ~8, the highest percentage of test takers achieve an 8, and each standard deviation is separated by a point such that a 9 in a section is one standard deviation above the mean. A 12 in a section is approximately a 95th percentile. If I recall, a 38 total score is 99th percentile. However, you can look up the exact numbers and the distributions at the URL I typed in earlier.
As far as what you should shoot for, I think the consensus is that at least 10 in each section, for a total of 30, is ideal. This is the average for medical school matriculants in the US. Of course, everyone wants to get a 45, and that should be your goal, too. But it is important to understand that a failure to meet this goal will not preclude you from reaching your other goals, namely getting into a medical school. Anything above a 30 is great; while scoring below 30 is not a death sentence, you will likely need to overcome that or mitigate it in some other capacity: through extracurriculars, by having superb letters of recommendation, a great personal statement, or some other "hook".