Remember that any notes or chords you include in a tune or your playing should have some purpose or function. The function of the 7th in a chord is like anything else: it's either structural or ornamental.
Structural use of the 7th would be how it works in voice leading. Typically the 7th is resolved down in the next chord that occurs. So if you are in the key of G minor and playing the C minor 7 chord (iv) the 7th of Cmin7 is a B-flat. If the next chord is a F9 then the B-flat goes to A (the major 3rd of F):
|--3--3--------------------------------|
|--4--4--------------------------------|
|--3--2--------------------------------|
|--5--3--------------------------------|
|--3-----------------------------------|
|--------------------------------------|
Or if you are going from Cmin7 to D7:
|--3--2--------------------------------|
|--4--1--------------------------------|
|--3--2--------------------------------|
|--5--0--------------------------------|
|--3-----------------------------------|
|--------------------------------------|
If I am working on using two chords (Cmin7 & F7 or F9) I will frequently work out possible inversions up the fretboard. A few examples:
|--3--3--|--6--5--|--8--8--|--11--11--|
|--4--4--|--4--4--|--8--8--|--11--10--|
|--3--2--|--5--5--|--8--8--|--8---10--|
|-----3--|-----3--|--8--7--|--10--10--|
|--3-----|--6-----|--------|----------|
|--------|--------|--------|----------|
In any event, look for the 7th of the chord to move down a half step or a whole step in the next chord. Ornamental uses are simply, you like the sound of any given 7th chord and will put it in regardless of how the voices work.
Hope this helps. Carry on! :)
