Ready-mix plants all around the country sell hundreds of different concrete mixes. Fine-tuning your mix and your techniques can get pretty complicated. But you can usually do good work if you just grasp the essentials.
Because I'm covering the basics here, I may oversimplify a few points. Experts in the industry could point out exceptions to many of the general statements I may make. But by and large, the ideas I'm about to explain hold true.