This is not an expensive hobby - almost everyone has a few pencils lying around! 
That said, it's good to recognize that there are various hardnesses of graphite in pencils, on a scale that goes from high "H" values to low H values, to HB, to low B values and finally high B values. The "B" side of the scale is softer and it's easier to make dark marks with these, but that softer quality also makes the pencil graphite more crumbly and fragile, and uses it up faster. It's also trickier to get fine points with softer pencil grades. For writing, the #2 pencil is adequate, but for drawing you may want multiple hardnesses. 
There are also charcoals and charcoal pencils, which we won't go into but which do deserve at least a passing mention. 
It's good to have erasers on hand (I prefer the white art erasers, made from a synthetic material, and to a lesser extent the 'kneaded rubber' ones that are gray.) There's also a lot to be said for pencil sharpeners - ideally reasonably high-quality electric ones with a holder that collects the shavings and doesn't send them everywhere on your table. A ruler would also be a good item to have on hand as it is great for clean straight lines if and when those are important. Once you've got your stuff set up and a surface to draw on, start drawing! 
Often my drawings are plans - rough composition tests for bigger artworks I intend to make in full color later. For me, often the simple pencil drawing is also great for experimenting with concepts for other things - plans for something I intend to create as a 3d model. Since I do a lot of 3d art for games and video VFX, this is common for me even if not necessarily applicable to most of you! 
HAVE A COMPLICATED 3D FORM YOU ARE TRYING TO DRAW? - Try breaking it down into component shapes like spheres, boxes, cones, wedges, etc. This can give you a way to figure out the shading in your mind - and then you can smooth it all out as you draw, so it looks more complex.

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