What and when are two great places to start. This answer will vary greatly depending upon if you are training for an athletic event or competition, or general fitness. You can split your routing into micro, meso, and macrocycles. (macrocycle is typically and entire training year, within this mesocycle lasting several weeks to several months, and microcycles which are typically one to four weeks.) In preparing for competition you typically have a hypertrophy/endurance phase(1-6wks), basic strength phase(1-2weeks), strength/power(around 2 weeks), and an unloading week to transition to competition phase(and many times these are tiered and repeated). For general fitness as Jason mentioned above it’s all about client adaptations and level of fitness, keeping in mind the body adapts fairly quickly to imposed stresses, and of course-switch it up to make the workouts fun and motivating!