Pick a pencil. Draw some lines. Straight; curvy; squiggly; thick; thin; wiggly; scribbly; zig zaggy; hatched; as many different styles as you can think of. Do some with a light touch and some heavy.
Draw the same styles again this time with as many different graphite pencils as you have to hand.
Add your work or photographs of it to your portfolio
(by the way - if you want to know about different grades of pencil - take a look at this )