Chains

Chains need oil. You have a lot of choices, like specially-made chain lube or a cheap quart of motor oil. Thick “wet” oils collect dirt but last longer and protect against rust better, while “dry” lubes keep the chain cleaner but must be reapplied sooner. It’s really a matter of preference, as long as you don’t use WD-40 (it dries up quickly, and the thin oily residue left behind will not protect the chain from wear).

Chain Installation:

New chains are typically too long for most bikes, so you’ll need a chain breaker to shorten it to your drivetrain’s optimal length. For a singlespeed drivetrain, install the rear wheel and loop the chain over both gears. Find the length of chain that would allow both gears to be directly connected, and use the chain tool to press out the selected pin. 

Don't press the pin all the way out, leave it sticking out of the outer plate so that you can reconnect it. A pin that’s all the way out can be impossible to put back in place.

You will use the chain tool to press the pin back in to reconnect the chain. You want the joined pin to stick out the same amount as the adjacent pins. 

Finally, press the pin out very slightly from the opposite side to loosen up the link that you just joined.

On a derailleur drivetrain you can find the optimal chain length by looping the chain over the biggest front and rear gears (not through the rear derailleur). Find the link that would connect them together, then add 2 links (one inch) from there. You can cut and resize the chain like the procedure above. 

Shimano HG chains require using a special pin to reinstall the chain, so for these you would push the old pin all the way out and use the special pin to join the chain. Many newer chains come with removable master links, so the chain tool is only needed to shorten the chain for these.