If you want to make a C6, just add an A note, the 6th tone from the C major scale, to a C major triad. A common fingering would be -

3
1
2
2
x
x

If you want to make a Cmaj7, add the 7th tone of the major scale, a B note, to a C triad. Fingering -

3
5
4
5
3
x

If you want to make a C7, add the 7th tone of the minor scale, a Bb note. Fingering -

3
5
3
5
3
x

9ths are a bit of an enigma. They're built on the dorian scale, I believe. Add the 2nd and 7th notes of the C minor scale to your triad. Like this -

3
3
3
2
3
x