Which kind of pencils should be used by a beginner for drawing?
I am not an expert but here is what I gathered from my experience.

Pencil have different hardness rated as follow:

Harder (very light strokes) 7H -> ... -> 2H -> HB -> 2B -> ... -> 7B Softer (dark strokes)


Start with a 2B or HB to "draw" (contour, shape, ...) 
Then add a 4B or 6B for your first shadings (I would go with 6B to widen the range of shade you can achieve). Do the lighter tones with your "first" pencil and the darker with this new one.
If you want "real black" instead of the grey provided by graphite pencil you can add a charcoal pencil.
completed with 3B, 4B, 5B etc. depending where you think there is a value (grey level) you can't achieve with what you have.