Play a major scale.
For example, C Major
8-C (repeats the first note)
The 7th note is B. If the chord was C Major 7, you would add the 7th note (B) to the Major chord (which is 1-3-5 or C-E-G). So, C Major 7 is 1-3-5-7 or C-E-G-B.
However, you asked about a "7" chord, which isn't a 'major' chord. '7' chords are played with a flatted 7th note added. So, a 7th chord is 1-3-5-b7 (ie: C7 is C-E-G-Bb).
It's actually pretty easy. Take some theory lessons - it's the basis for all the music you will play.