Penn Station borders 7th Ave. & 8th Ave. 7th Ave. heads downtown (south), 8th Ave. heads uptown (north). There are cab lines on both avenues (D400 says no taxi stand on 8th--there is an attended taxi line). Decide which one you want and follow signs (or your sense of direction, which can be tough inside Penn Station), or ask someone, "Which way is 7th Ave.? (or 8th Ave., as the case may be)" and go that way. Take escalators that go up--Penn Station is basically underground 2 flights and you have to go up to the street--or the poorly located elevators; if you need them you will have to ask.
A nice feature of your LSL arrival (ha!) is that the scheduled arrival time, 6:45 PM I think, is precisely in the midst of evening rush hour, so you will be weaving in between hordes of commuters going in the opposite direction (mostly to the LIRR, some to NJT or Amtrak.) On the other hand, you may well arrive hours late, and thus miss rush hour. By the way, if you are not familiar with the layout and prone to get lost, I would not recommend making a B line (rushing) in Penn Station; go slow, check signs, and keep asking questions and correcting your course.
Finally, the above advice is only for tourists, who will need to stand in long cab lines to get a taxi. New Yorkers by-pass the lines as follows: leave the station with your luggage on the 7th Ave. side, walk south half a block, cross one street (31st St.),, stand on the curb (there is a bus stop there), and hail the first passing cab--thus by-passing the long cab line in front of Penn Station. (Btw if you're heading north you just tell the cab-driver you're going uptown). This works for me, 100% of the time. Never waited in that cab line.
Sorry the answer wasn't more direct, but NYC (incl. Penn Station) is actually a rather complicated place. And getting a cab is the quintessential NYC transportation move.