I usually see 1...d5 or 1...Nf6. I'm not sure what the proper response is, but those are what I usually see.
What's the common reply to zuketort (1. Nf3) ?

I've never heard of it referred to as the "Zuketort opening" -- usually that's in reference to the Colle System, isn't it? So that's probably part of your problem in finding information about it.
The other problem is that 1.Nf3 doesn't really have one specific name -- it's a pretty transpositional opening, depending on subsequent moves by both sides. Coupled with an early c4 vs. black d5, it's the Reti Opening. Early c4 vs. black c5, it's a variation of the English opening. Fianchetto the king bishop after g3, and you may get a King's Indian Attack after d3, or probably a Catalan after d4 & c4. With other continuations, you may get any variety of queen pawn games, half-open e4 defenses, almost anything. Nf3 is played in many, many openings, which is part of the appeal of it as a first move -- it's flexible & noncommittal.
If you're looking for advice on how to play *against* it, you can usually follow the same types of ideas & plans as you would against a d4 opening.

thanks guys, the reason I called it the zukertort opening is because chess.com game explorer lists 1.Nf3 as zukertort opening.
It is the "Zukertort..." Like Echos said, check the game explorer. If you want, I could play a few practice games with you to help familiarize yourself with the opening.

As to the nomenclature, I think it's the Zukertort until BLack actually makes a move, similarly to how 1. d4 is a Queen's Pawn Game until something else happens.
I typically reply Nf6, though I've seen and played d5, c5, and on some infrequent occasions, c6.

I reply f5, and if he doesn't play d4 follow that up with c5. Don't think it's in any books, but it works for me sometimes.

In the past, the Zukertort Opening was the name for 1.Nf3. Here's a quote from The Sun, New York, 1891: "The turn to open the game yesterday belonged to Steinitz, and he showed his pertinacity of purpose by again playing Kt-K B 3, known as the Zukertort opening, which, practically speaking, after a few moves became a Queen’s Gambit."
Neither Modern Chess Openings nor Fine's Ideas Behind the Chess Openings gives any name to 1.Nf3 by itself--in modern texts it's a nameless transpositional weapon.

there is a game explorer under learn, click that and you can put any move in and the most popular response will show up
1.Nf3 can transpose into all sorts, because the most logical square for the king's knight is f3 in virtually all lines of all openings...You might get some milage out of researching the reti opening, however.
Usually players choose the move that fits their standard defense to 1. d4. QGD players choose 1. ... d5, Indian defense players choose 1. ... Nf6, Dutch players 1. ... f5, etc.
But this doesn't work well for a few openings so do your homework.
Players who like the Sicillian and the 1. ... c5 English can play 1. ... c5.

What's the most common reply to the zuketort opening 1. Nf3? I can't seem to find any information on it. When I encoutered it, it really threw me off. Any suggestions help would be much appreciated.
Why do u put a question mark to 1.Nf3? it is perfectly playable;this first move propose to the second player using several opening choices: open, semiclosed and closed systems.Regards.
What's the most common reply to the zuketort opening 1. Nf3? I can't seem to find any information on it. When I encoutered it, it really threw me off. Any suggestions help would be much appreciated.
Why do u put a question mark to 1.Nf3? it is perfectly playable;this first move propose to the second player using several opening choices: open, semiclosed and closed systems.Regards.
The question mark is there because the sentence is a question.
Anyway, I agree with all the others that 1. Nf3 can turn into many different openings. It can also turn into the same openings as if you start with 1. e4. That can happen if black after 1. Nf3 plays 1... Nc6 and white follows with 2. e4.

The most common replies are d4 and Nf6. I like to play d4 and sieze the center, but Nf6 develops a knight. Both are reletivally solid moves.
1.Nf3 can transpose into a Barcza Opening, Queen's Indian, or a host of other openings. The most usual response is , I believe, 1...d5 or 1...Nf6. These both offer Black good chances, but I prefer 1...d5. Much more dynamic for me.

What's the most common reply to the zuketort opening 1. Nf3? I can't seem to find any information on it. When I encoutered it, it really threw me off. Any suggestions help would be much appreciated.
Why do u put a question mark to 1.Nf3? it is perfectly playable;this first move propose to the second player using several opening choices: open, semiclosed and closed systems.Regards.
The question mark is there because the sentence is a question.
Anyway, I agree with all the others that 1. Nf3 can turn into many different openings. It can also turn into the same openings as if you start with 1. e4. That can happen if black after 1. Nf3 plays 1... Nc6 and white follows with 2. e4.
It' Miller time!!!
What's the most common reply to the zuketort opening 1. Nf3? I can't seem to find any information on it. When I encoutered it, it really threw me off. Any suggestions help would be much appreciated.