How To Draw Realistic Apple In photoshop:

1. Make a sketch of apple with hard round brush with 100% flow and opacity. Turn on the Shape dynamics with control of pen pressure and turn on the transparency with control of  Pen Pressure from brush panel.
2. create new layer under the sketch layer and fill with solid color
3. create new layer at top of solid color's layer and add the shading and tones on the apple structure.
4. Mix those colors
5. turn off the shape dynamics of brush panel. Change the opacity to 60% and flow to 20% of brush property. Then mix those colors more smoothly. 
6. Turn on the shape dynamics again and make the texture.
7. add fill light with blue color tone at the edge of that apple.
8. Create a new layer on the top and fill it with 50% grey. Change the layer mode to overlay.Then use black color for more dark tone and white color for more bright tone with soft brush ( very low opacity and flow) on that overlay layer.