I'm a beginner. Should I mix the thinner and oil paint before painting, or can I use the paint directly from the tube?

When you're sketching out something on your canvas, you can mix a little bit of thinner in your oil paint so that you can wipe it off if you make a mistake. Once you begin painting, stop using thinner and use the paints directly from the tube, or use a medium like linseed oil. Sometimes if the paint is a little bit stuck, you can dip your brush in a tiny bit of thinner and mix it with your paint. What I usually do is keep a jar of thinner and a jar of medium (linseed oil). Usually, thinner ends up being used just to clean your brushes in between colors. To do so, just dip your brush in, and then wipe with a paper towel. You will still need to wash your brushes after you finish, though.