This is a picture of my wife's piano that I fixed up for my guitar students. I have marked out which notes correspond to the guitar strings in relation to "middle C".
The middle C note is usually pretty simple to find on a piano or keyboard. Sometimes, there is a marking that helps distinguish it. (like an arrow that points to the key)
If there is no marking, then look at the pattern of black keys.
Notice that there are groupings of 2's and 3's. Find a grouping of 2 and locate the white key to the left of the 1st black key in the grouping (also see the arrow in the photo below).
This is a "C" note.
Now, find the "C" that is in the middle of the keyboard, and that should be "middle C".
In this style of tuning, the "ear" will need to be utilized.
In other words, the sound of the note on the piano is used to get the right note on the guitar.
(Acoustic pianos, uprights, etc. can be out of tune as well, so just keep that in mind)
Here are the basic steps:
1. The letters from left to right on the piano are E-A-D-G-B-E. These correspond to the guitar strings going from the thickest string to the thinnest. They are numbered: 6 (thickest)-5-4-3-2-1(thinnest)
2. Play the note marked E on the keyboard and listen to how it sounds. Now play the guitar string "6" which is also E. Repeat this a couple of times and see if the guitar note sounds higher or lower in comparison. If it's lower, then tighten the string. If it's higher, then loosen the string. Repeat this until they sound as close in pitch as possible. Ideally, they will be the same pitch when you finish. (this process can be really easy for some and close to impossible for others, so don't be too hard on yourself)*
3. Now repeat this process for each of the notes from left to right (E-E) on the keyboard with its corresponding note on the guitar (6-1).
*You can check yourself with an electric tuner, until you are comfortable enough with the process. Of course for some people, if you have an electric tuner why bother, right? Well, for what it is worth, I think it is a very valuable skill.
If you have a guitar teacher, take his/her advice on these topics.