It depends. For stuff like layering, washing etc where you need really thin paint I prefer thinner medium, but for base coats and in the beginning, water will suffice. But use clean water.

Haha, it's tricky for sure! But, as with all things, it just takes a bit of practice.

I just use water yeah, but I think there are other products out there that can help thin paints down further than just water called "mediums". I don't use them personally and I'm by no means an adept at painting 😅