Hi Grace! Hopefully you're referring to college, as that's what most of my tips are based around.
At my university, we often hear "C's get degrees", which is a statement saying that in many cases, simply getting the degree matters much more than GPA; C's are nothing to be ashamed of. That said, here's some tips from a GPA-mindful student like myself:
1. Study. Seems basic, but a lot of people just don't, thinking they can pass regardless.
2. Pick the right teacher. Using the website RateMyProfessors really helps in deciphering what you can expect from a professor. I'd say course difficulty is 60% the material (Calculus is harder than Psychology 101, trust me) but 40% which lecturer you have.
3. Ask for help when needed. Many people are shy about visiting professors, but usually you not only understand the material better but look good to the professor by showing you care about the material.
4. Find out how you best study. Some people need notecards, some recopy their notes. I personally do best when I can access practice tests or test reviews.
5. As for finding a job, if you have the degree, being a "C" student shouldn't harm you much. When job hunting, stress your skills, be they professionalism, communication, science, etc.