A workout for mass gain aims to micro damage your muscle fibers. That is to obtain muscles hypertrophy as a consequence of the repair process coming into place when you rest. Although the structure of a mass gain workout is very specific it doesn't work alone. Do remember that the way you execute your exercises is crucial as well as your diet as explained in the nutrition quick start guide linked further below.
An appropriate workout meant to gain mass will ensure each and every muscle to execute at least 60 repetitions per week at such weight that you can lift with proper execution. In technical terms it will be an high volume workout due to the total weight lifted by each muscle group through each exercises. To keep the concept of volume simple, with 60 reps of 10kg you'd have a volume of 600kg.
Ideally any workout should last around an hour or less for many reason. If that is not the case, it is convenient to have a supplement during your workout to take after the first 45-60 minutes, this would prevent your body to start feeding from your muscles. Because your intent is to gain mass you shall also think to a pre and post-workout meal, the first with the nutrients giving you energy and the second meant to re/build your mass. This said it is unlikely to have so many reps on your whole body in just one session unless leveraging on compound exercise which would be a good strategy though the more you progress and more you'll want to increase volume on each single muscle. That is the reason for which a mass gain workouts is often split in days focusing on different muscle sections.
As anticipated, the right workout will not do the job for you. A proper exercise movement is what will cause micro damages to your muscle fibers together with the right weights. Every exercise has a first part of the movement which is to reach the target position, this part of the movement will be quick and powerful to be executed in about 1 second. The second part of the exercise is to hold on the target position when this means to resist to the weight or squeeze and that goes for about 2 seconds. Finally you return to the initial position and this is where your muscles start to really burn while opposing resistance to the weight. Most of the micro damage to your muscle fibers will happen in this last phase. Execute this last movement slowly in not less than 2 seconds but it can be even 4 or more seconds depending on the total number of repetitions to achieve in about 40 seconds. This will be specified in your workout.
The weight will be the maximum you can lift while staying within the rules above. If unsure starts with something which most certainly work for you, then change it at next set based on how easy or difficult it was to execute it properly. Keeping record on My Fitness Page will help you to reach the right weight slightly above the borderline of your comfort zone. You'll seek to increase the total volume at each session by either adding a small weight on the last set or at each set or an additional repetition if you're reaching a plateau point. If you're unable to measure any further improvement for a few sessions it is time to change workout.
Return to My Fitness Page and choose your workout. If you come from a period of rest or you're just starting then you should take it easy 1-2 months with low weights in order to condition your body and educate yourself on the right movements. Any injury would stop you from training for weeks or even months, it's not worth the risk.
Do not despair though. New starters have a bonus as their body gets shocked by the change of lifestyle and improves quickly at beginning.