NYC Subway help! Penn Station to Union Square?

Trying to figure out which subways to take is like trying to understand astrophysics to me. I need to go from Penn Station to Union Square, and I'm looking for some inside help on what are the best subways to take, preferably from someone who is very familiar with NYC subways. A little detail about where the subway entrance is would be helpful as well. Like, in order to get on the subway will I need to get out of Penn Station and find it on the street or will it be in the station with the other subways? I probably sound really dumb, but reading subway maps is like reading gibberish. So confusing! Thanks!