Sour dough starter in baking bread

The recipe should tell you how much starter to use. A lot of recipes I use go by baker's percentages (comparing all other ingredients to the amount of flour needed for the final dough), and call for ~20% starter to the weight of flour. So if you're making 1 loaf that calls for 500g of flour, you'd use ~100g starter.

And yes, you want starter that has been fed and active, meaning you've fed it, and given it time to ferment (it should also rise and look spongy and airy). Ideally you'd take out what you need for baking before it's "fallen" or collapsed on itself again. For my starter, I let it double in volume, but don't let it go much beyond that because in my experience it collapses soon after doubling. Hope that helps!