Coming back into fitness after a long time or is it your first time? The first 2 weeks aims to getting a feel for your weight and reps with a warm up related to the muscle group of the day and a cardio session afterwards. Ideally you should go for very low weights and high reps at first, increasing the weight for the next set while reducing the reps, this is to help build stamina. Start with 18 reps with a low weight, aim for 4 sets and decrease the reps by 3 for each set and increase to the next weight. Use this opportunity to perfect your form, to do the exercise properly. Slow and controlled movements, tut (time under tension) is your key to a stronger body. From week 3 it's time to increase the limits. Reduce the sets to 3 and start your first reps at 15. Use the weight from the first 2 weeks you used on the 12 rep as your starting 15 rep weight, so when you get to the 9 rep set you're using a new heavier weight. If at any point you get to failure, do not move on to the next higher weight until you can comfortably finish the set the week after. Hopefully when you finish week 5 you should be able to finish all your sets comfortably. If you're finding lower weights too easy to begin with, eg you have finished the 18 or 15 rep set without breaking a sweat or feeling the muscle burn, increase the next rep weight by a factor of 2 or 3 (skip the next weight and go to the higher one again, or skip the next 2 weights, only you can judge this). Remember to make a reasonable adjustment to the first rep the week after. After week 5 you may continue to run this routine but drop the reps to 10, 8 & 6 and your starting weight at around 80% of your 1rm (eg if your 1rm is 10 then start with 8).