The weight you choose largely depends on you, and if you have ever used any weights before. If you haven't, then you'll likely start off fairly light, as opposed to if you're accustomed to lifting weights. Start with a weight that you can complete the reps with but you feel tired by the end of the set - not totally exhausted, but well-used. For some exercises, you might find that you have to use a lighter weight and that's fine. Half-way through th programme, the weights you began with you now find a little too light so you want to use a heavier weight. Experiment, see what works for you, and change it if/when you want.