Like the title says, I’m visiting NYC in March. I know some of the places I want to see, (Natural History Museum, Empire State Building, etc) One question I have is about dining out as a single person. What time in the evening is best? Should I make a reservation? I can find out about how to get around, and places to see, just not about this stuff.
I can’t think of a place you’d need a reservation. Outside of something like Friday or Saturday at 8 PM. I’ve walked into the Russian Tea room right on 57th Street at those times and gotten a spot.
As far as the Empire State Building, to avoid long lines at peak tourist times, you could consider going to the top of Rockefeller center. 30 Rock. About 800 feet high. From there you can see the ESB, Chrysler Building, Pan Am/Met Life and One World Trade Center.
Have fun! 
You can often eat at the bar if you’re dining alone but prefer company. I don’t think you’de have a problem on weeknights, but less-busy dining hours are before 6 and after 9.
Reservations may be needed at popular spots during prime hours friday or saturday. Do you have any places in mind?
Eately on 23rd street/Broadway has a whole bunch of different mini-restaurants, its great for single diners since you can usually sneak into a single seat at the different spots’s counters without a wait. The pasta restaurant and the fish restaurant are both great, if a tiny bit pricey (not outside the norm for NYC). Italian Restaurants - New York Flatiron district | Eataly
ETA: The ESB is a definite tourist trap and can take a very long time as well. Top of the Rock is a better view IMHO, or, another option is a couple of drinks at Sixty-Five, the Rainbow’s Room’s separate, non-formal-dress bar/lounge. The elevator’s free, just pay for some booze. A better use of $30 if you ask me.
Just eat the Halal chicken from the street carts every night. You won’t regret it!
We were there last March for my daughter’s 18th birthday - and actually, places near the Theater District may well be crowded enough that reservations are helpful. We wanted food after a show and had to wait a half hour for the noodle place we went to in Hell’s Kitchen.
Do not eat in Times Square. The places right there are hugely overpriced and/or tourist traps. Who on earth would go to New York City and eat at an Olive Garden??? 
The Hell’s Kitchen neighborhood isn’t hellish at all, and is just 1-2 crosstown blocks west, and has tons of great restaurants (though as noted, right after theater times they might be crowded).
NYC also has tons of other interesting restaurant concepts, e.g. one devoted to peanut butter, another based on mac and cheese (which I couldn’t persuade Moon Unit to try…); we saw one that had mostly dolled-up oatmeal…
Tons of cute fooderies in the West Village. A shop selling only macarons. Another selling gourmet ice cream bars. Restaurants of every ilk.
Little Italy is fun for food too - we make a point of going to Lombardi’s (coal-fired pizza oven) though you HAVE to go there a bit before normal mealtimes or they will have a line out the door (and they don’t take credit cards).
The weather may be great, or awful. It was miserably cool and rainy our first day there last year; the next day it was perfect light jacket weather. We had the surreal experience of walking around Central Park and feeling warm, while spotting a pile of the last of the snow from the winter, and skating in the rink.
You will walk. A lot. Even if you take the subway everywhere you can walk several blocks underground to get to the train. The subways are not handicapped-friendly - escalators are rare; supposedly there are elevators but I walked many blocks underground at Times Square following the signs and never saw it (I was dealing with a bum knee at the time so honestly needed the elevator).
I could spend a month solid in NYC eating at a different restaurant for every meal and doing little else.
I did the “Food on Foot tour” that was a part of a museum pass I got. It was awesome. I would do several if I had time.
I also really like the roast beef sandwiches I got from different delis.
I did a tour of the tenement museum which was cool , the Morgan Library - Awesome, and The metropolitan art museum, MOMA, and a bunch of others. I can’t wait to go back again and see more.
Also as part of the museum pass I did, was a Hop on Hop off bus tour. It was fun and I could use the bus to go to different parts of the city for no additional cost and with a better view than the subway provided. I think it was “City Pass”
The top of the Rock had a long wait when I was there.
oh and I am single and never had any issue with being alone.
I think the ESB is worth doing. So iconic. All those old movies done there. Suggest going first thing in the morning, yes the lines will already be long but they will only get worse. And the air is much clearer in the morning. Another good time is late in the afternoon, around 5:30 - the lines tend to subside a bit then.