86 Street Station
The 4/5/6 lines stop at 86th st, 76th st, 68th st, 59th st, 51st, 42nd st, 34th st, 28th st, 23rd st, 14th st (union square ) Astor place, bleecker street, spring street, canal street (China town) and the brooklyn bridge
This is your main hub into the city. If you are looking to go downtown, take the 4,5,6 (the green ones) to Grand Central. From there you can transfer to other lines.