There are a number of 7th chords available. This post is concerned with the following three types
Major 7th
Minor 7th
Dominant 7th (this is the one most guitarists know when they're asked to play a 7th chord i.e. C7)

Major 7th
Going back to my earlier post on major chord construction you'll see that
major triads are made up from the 1st, 3rd and 5th notes from the major scale. 
To create a major 7th chord you simply add the 7th note of the major scale.

So using the good old C Major scale as an example. You get
C (1st note of the scale)
E (3rd note of the scale)
G (5th note of the scale) and
B (7th note of the scale)

Therefore the scale spelling of a major 7th chord is 1, 3, 5, 7

Dominant 7th
Slightly different from a major 7th chord. This chord uses a flattened/minor 7th note. So in the case of a C dominant 7th (or C7 to you and me) you get the notes.

C (1st note of the scale)
E (3rd note of the scale)
G (5th note of the scale) and
Bb (flattened/minor 7th note of the scale).

Therefore the scale spelling of a dominant 7th chord is 1, 3, 5, b7

Minor 7th
This chord is an extension of the minor triad and as such is based on the natural minor scale and uses the 1st, 3rd, 5th and 7th notes from that scale.

C (1st note of the scale)
Eb (2nd note of the scale)
G (5th note of the scale)
Bb (7th note of the scale)

Therefore the scale spelling of a minor 7th chord is 1 b3, 5, b7 (don't forget scale spellings are relative to the major scale not the minor scale).

So if you review my other posts on major scale construction and the circle of fifths you should (in theory at least - pardon the pun) be able to work these chords out in any key.

We'll discuss diminished 7ths later when I cover diminished chords.

The C Major triad is made up using the standard formula for major triads. In this case the 1st 3rd and 5th notes from the major scale.

The notes in the C Major scale are

C D E F G A B C

Therefore taking the 1st 3rd and 5th notes from this scale gives the triad notes of C, E and G

Taking this one step further if you look at the first position C Major chord you have the following notes

5th String 3rd Fret = C
4th String 2nd Fret = E
3rd String open = G
2nd String 1st Fret = C
1st String open = E

So you can see that the open chord fingering doubles up on the C and the E notes but keeps within the triad note structure.