Last time I was in New York I found it difficult to navigate the subways as I didn't realise some stations only go uptown or downtown. I am staying at 59th, is there a station nearby that goes downtown.

All subway stations do go both ways, but not necessarily form both sides. The station at 50th St has seperate entrances for each direction. If an entrance is only for one direction, there will be a very lasrge sign over the entrance stating that. Ex, 86th St 1/9 UPTOWN ONLY on the overhang as you walk down the stairs.
That said, all of the 59th St stations I can think of are totally accesible from either side. Where on 59th are you staying--east or west? That will decide for you which station you use.

The stairs descending to a subway station are clearly marked UPTOWN ONLY or DOWNTOWN ONLY (if necessary). If you need to go downtown and you're on the uptown stairs, cross the street.

For example, the 4-5-6 trains run on Lexington Avenue. The DOWNTOWN ONLY entrances are on the west side of Lex, and the UPTOWN ONLY entrances are on the east side.

As stated above, many entrances service both uptown & downtown and obviously won't be marked.

As indicated in the other replies it will indicate UPTOWN OR DOWNTOWN at the subway entrance. If neither is mentioned you can access either the uptown or downtown train at the station it is the 59th ST station you are near you have access to uptown and downtown trains as well as Queens trains.

59th St. has one of the more major stations.... many trains stop there going both ways. You'll be fine.

Don't forget to get a free subway map from any train station booth. If you are going to use the subway much or for several days, I'd get the $17 unlimited ride 7 day pass.

The Bronx is UP and Brooklyn is DOWN. Also remember only a couple of trains are crosstown.

If you want to go downtown, get on the trains that say To Brooklyn. Never get on a train that says To Queens or To the Bronx.