Generally you start by applying to the college by filling out the application and submitting other documents, like a highschool transcript. Then it takes some waiting while the school processes it and decides to accept the application.
When accepted, and I think Columbus State is open enrollment so you can't really not get accepted, anyway when accepted they'll have you take an assessment/placement test that will help determine where you stand. A person that just graduated highschool likely will start with "college level" classes, however schools offer classes that are below that level for people that need review or brushing up, etc., if they have forgotten highschool lessons which is common if their job didn't require that knowledge or skills.
Credit hours are roughly equivalent to the amount of time spent in the classroom. A 3 credit hour class might consist of 3, 1-hour long class sessions each week. A general rule of thumb is that the time spent outside of class for reading, studying, and doing homework is equal to about 2 hours for each credit hour. So a 3 credit hour class might take 9 hours each week (3 in class + 6 (3x2) hours outside). 12 credit hours is considered full time enrollment (12 + 24 = 36)