What to do:
1. Start easy.
If you've never done weights before, start with a weight that feels easy and do a few repetitions when you focus on your technique. Gradually increase the weight until it starts to feel challenging. As soon as you have to slow down to be able to continue with the exercise, you've got the right weight. Use this weight for your first set.
2. Know when to increase the weight.
If you can do more than 20 repetitions, it's time to lift more weight. See how easy it is to lift the weight and how you feel when you've completed your sets. If the last few repetitions were quick and smooth, you can increase the weight a bit. If the last few repetitions were slow and felt heavy, you have the right weight.
3. Find out how much weight to add.
When you increase the weight, do it in baby steps. Increase the weight by between 2.5 and 5 kilos each time. You need to strike the balance between pushing yourself and listening to your body. Sometimes you can make a lot of progress in a short space of time, other times it takes longer before you can increase the weight again. This is to do with your level and experience. The rule of thumb is always that you are able to maintain a good technique. If you can maintain a good technique and the last few repetitions weren't sufficiently challenging, you can increase the weight!