You take it at the beginning to get the potential alcohol volume of your finished beer. You also take it before bottling to see how much of the fermentable sugars were actually fermented. Then you can be sure that fermentation is actually done, and your beer is ready to bottle. You can also determine the actual alcohol volume of your beer. A hydrometer is not essential at this level, but it s a good thing to get used to.
Caps are cheap, and available at any homebrew shop.
The standard, simple capper is a two-handed device that resembles one of those wine corkscrews with the arms off the side. You put the bottle on a steady surface, place the cap on the bottle, and then clamp the capper down on top, crimping the cap in place. You need two hands and a steady surface for this one. Some people prefer a bench-top capper that you only need one hand and a steady surface for. I ve never used a bench-top capper.