Here is what I use to practice sketching

• Drawing progressions: Betty Edwards' "Drawing on the Right Side of the Brain". Truly gets you most of the way there (I like the 3rd edition more than the 4th edition). I did maybe 1 drawing per week (30min) for half a year, at most, before staring this.
• Pencil and eraser, black drawing pen
• Some markers in very light shades (I'm using the "Neutral" set from Spectrum Noir). Light colors are lovely for sketches. They deemphasize lines that went slightly wrong, just like when drawing with a pencil.