Don't stress.
Firstly, the primer is just so the paint grips the mini. If it is worn away, nothing too awful will happen. If there are finger prints, take a little nail polish remover (and I mean a little!) and gently wipe away the finger prints. Use a paintbrush or a q tip, with slow and small strokes.
Secondly, don't feel bad if your first work is not the best! Enjoy the action of putting the paint on, and coating your models. Get a feel for it. Nobody's perfect first time, and the less you pressure yourself, the more fun it is.
Remember these are effectively driving lessons, you're allowed and encouraged to learn and make mistakes. Nobody will die.
Thinning paints, you need a palette. A non porous surface like a tile, a plastic lid or a small plate nobody wants. Dab water in one brush full at a time. If your brush is too wet, wipe it on a paper towel before putting it on the figure.