How would I susbtitute fresh spinach for frozen?
Frozen spinach has been boiled/blanched. You can do this if you really want it to be as much like frozen spinach as possible, but really, you can just cook it with the water left on the leaves from washing - effectively a bit more like steaming. There's no need to freeze it. This will result in something with fresher flavor and a bit more substantial texture than frozen spinach (not as eager to disintegrate). And you don't have to worry about moisture much - a lot of the water will cook off - but you can certainly drain it additionally if it's too much for you.

If you really want it exactly like frozen spinach, you could boil it, and maybe even freeze it to help mess up the texture, but I'm guessing you'd prefer cooked fresh spinach anyway. In this case, you'd have to squeeze and drain it just like with frozen spinach to get the water out. (If it's unclear how to do this, see What is the most efficient way to squeeze water out of cooked spinach?.)

Based on nutrition facts for raw spinach and frozen spinach, one 10 ounce package is approximately the amount you'll get from cooking a 340g (12 ounce) bunch of spinach - that weight is probably after removing the stems you're not actually going to cook, though. This fits with my experience cooking down spinach. Most things you'll use it in are really forgiving, so probably best to err on the larger side with your bunch of spinach!