Exercise your eyes
When looking at a painting, as artists, we need to look at the light. How the artist depicted it. Just like a cook tastes the food at the restaurant, guessing the ingredients, we need to learn and understand that light is what gives dimension, life, and splendor to our work.
Forgive the watermarked picture because I really wanted you to study this painting, and exercise on it. Try to shade on your paper all the shaded places, and see what it will give. The result is not critical. What is important is for you to trace back all the nooks and curves that will come out, just by blackening the paper.
If Caravaggio is too dark for you, you can study Sorolla's painting, but it will be much harder for beginners to discern the several nuances and shading of white.
You can find more masters, here.