There are two classes of beginner who want to build muscle:
Those who are looking to look good for the beach/club ASAP
Those who are in this for the long run and want to make lasting changes in their appearance.
Sadly the former is the most frequent. It's the guy who only want to do arms, chest and delts and only cares about getting a pump to "look swole". Honestly I personally refuse to work with these clients. They normally have unrealistic expectations about how much and how fast they can gain, do not really want to put in the hard work required to progress and are likely to quickly quit training or be on and off.
With the later the first 6-8 weeks of training should NOT have for a goal to increase muscle mass or strength (although it will happen). It is TO LEARN TO LIFT. Learn then practice and finally master the optimal form for big compound movements like squats, front squats, military press, bench press, barbell row, deadlifts, chin-ups (band assisted). NEVER allow a bad rep... NEVER increase the weight if it leads to form breakdown. Only use a weight that can be lifted with proper from and without grinding.
I prefer more sets of fewer exercises so that the client can get more practice on each of the key lifts.
Each of the selected "big lifts" should be practiced 2-3 times per week.
It is of primary importance that your client understands that he must be patient:
Patient about taking the time to learn to lift
Patient about adding weight to the bar... during those first 6-8 weeks they should only add weight when it is necessary because the weight is feeling too easy. Have them increase the weight because they have to, not because they are capable of.
Patient about adding exercises.
They must become perfectionists about perfecting lifting form.
There are basically 4 levels of effort when you are in the gym:
REHAB/PREHAB -- PRACTICE -- TRAINING -- TESTING
These are in order of the intensity (amount of effort relative to your maximum effort).
During the first 6-8 weeks a beginner should see most of his training as "practice"... the goal is to get better at the movements, not increase performance (although their performance and size WILL increase because they are starting from scratch). When you begin, the longer you stay in "practice" mode, the better your foundation for future gains will be.
For someone who is in this for the long run, 6-8 weeks is nothing.
New personal trainers make ONE typical mistake: they feel the need to show how much they know and prove themselves. As a result they tend to design programs that are way too complicated for the needs of their clients and also have their clients add weight too fast (to be able to say "the program worked great, you added 20lbs more this week").