It costs $2.75 each time you use the subway, and there are metrocard stations located in every subway next to the entrance which makes it very convenient to get your card and re-load it.
How to Navigate the Lines
There is an abundance of maps online, and Google Maps is up to date on all the lines, arrival times, and it is very good at navigating which subway entrance to use (which I find to be the most confusing part).
The pictures below show an example of how Google Maps can help navigate the subway. The directions below are from Coney Island Beach to the Empire State Building.