Lightly draw out cactus shape and an uneven line midway behind it. Draw the spikes as dots in the front, straight lines at the sides and slanted lines in between getting steeper going out, to transition to the straight lines. The more angles between the dot and the horizontal, the more round it will look. For shading you can draw horizontal lines all up closet to each other, which is called hatching. If you want a deeper section of shading do the lines on top in the other direction, making it cross-hatching. Draw a circle for the sun somewhere and imagine lines coming onto your cactus from it, and shade where the light isn't hitting.