As I undergo my own personal training certification - to make sure I'm giving all of you angels sound advice - I've come across some mistakes in my own training. Mistakes that I want to be frank about so that you can all get the optimal education and training you desire!
I didn't realize that rep ranges were designated to a certain overall fitness goal. For instance, if you're looking for size and strength, you should be performing different rep ranges from someone looking for power or endurance. Here's what the correct ranges look like with relation to your fitness goals:
20-25: fat loss
After you pick your desired outcome (i.e. strength) you have to choose a weight. The way you choose a weight is by figuring out how many reps you can perform to failure (i.e. when you can't perform anymore) then deducting a few reps so you can repeat several sets. For the strength trainee, training in the 6-8 range, you will likely be using a heavier weight. It's important to break in between sets to allow your muscles to take up oxygen and sugar again, and to prevent lactic acid build up from causing contractile failure too early.