There are three types of 7th chords. The dominant 7th, major 7th and minor 7th. All of these chords are based on triads.

The dominant 7th chord (eg. D7) is based on the major triad. Once you have built the major triad from the major scale, you add on the flat 7th. If we are using D as an example this note would be C. This is because C# is the major 7th of D Major and so the flat 7th would be C. The D7 chord would therefore be D F# A C. Here is the chord in tab...

D7
e|---2-F#--|
B|---1-C---|
G|---2-A---|
D|---0-D---|
A|---------|
E|---------|

The major 7th chord (eg. Dmaj7) is also based on the major triad. Once you have built the major triad you simply add on the major 7th. With D, this note would be C#, because C# is the major 7th of D Major. The Dmaj7 chord would therefore consist of D, F#, A and C. Here is the chord in tab...

Dmaj7
e|---2-F#--|
B|---2-C#--|
G|---2-A---|
D|---0-D---|
A|---------|
E|---------|

The minor 7th chord (eg. Dm7) is based on the minor triad. Once you have built the minor triad you add on the minor 7th. With D, this note would be C, because C is the minor 7th of D Minor. The Dm7 chord would therefore be D F A C. Here is the chord in tab...

Dm7
e|---1-F---|
B|---1-C---|
G|---2-A---|
D|---0-D---|
A|---------|
E|---------|