Updated: May 10
Here I break down the basics method to building muscle. It's simple but it isn't necessarily easy. To build muscle you need to force your body to cooperate. There really isn't any need for your body to expend energy growing muscles if they aren't useful. Your body will happily store fat because it doesn't require much energy to store it. It basically just sits there wobbling. But your muscles require constant feeding and while it is now easy to stuff food in our mouths it is only recent history that has given us an abundance of calories. For most of human history our entire lives were focused on not starving to death. So, if we want big muscles we need to 'trick' our bodies into thinking 20 inch biceps are necessary.
If you lift a 10kg weight over your head it is your muscles that are doing most of the work. If it's easy your muscles won't need to change, they just do it with the muscles fibers they have. However, if it's difficult your muscles will need to adapt (presumably by growing) until eventually it is easy to lift the weight, at which point they will no longer need to grow. If you want your muscles to continue to grow you need to either lift the weight over your head more times in a row (more reps) or lift a heavier weight.
This is the basic principle of weight training. You need to stress your muscles (by adding resistance) to force them to adapt (grow). It is called 'hypertrophy' which means to damage your muscle fibers which encourages them to grow back stronger.
The main way to increase the stress on your muscles is to either increase reps or increase the weight. Both are great things to do, however, they are different. For example, imagine you are choosing to either lift a 1kg weight over your head 100 times or a 100kg weight over your head once. In which of these two tests do you think large muscles would be more useful? The very light weight mainly tests muscular endurance while the very heavy weight mainly tests muscular strength.
If you think about endurance athletes like cyclists and runners they tend to have small muscles while strength athletes tend to have big muscles. So to build muscle size, we want to be training with heavier weights and closer to the lower end of the rep range. This is where big muscles are more useful so naturally if you force your muscles to adapt to lifting heavy weight they will likely react by growing bigger. There are furious debates around the ideal rep range but the science is telling us that training from 5 to 20 reps works if you want to build bigger muscles.
So, a simple muscle building workout plan includes both increasing reps (up to 20) and/or increasing the weight. For example, on day 1 you lift a 10kg weight over your head ten times. Day 2 your option is to either increase the weight to 11kg and lift it 10 times, or lift 10kg over your head 11 times. There is no right or wrong. Both are effective.
I would suggest if your goal is to build bigger muscles then investing in some weights would be useful.