Adding additional lights to a circuit
&quot;It keeps tripping&quot; means you have a short somewhere. That being said, you need to have all wire 12 AWG on a 20A circuit. Since you now have some 14 AWG wire, you need to replace the 20A breaker with a 15A breaker.
My best guess is that you have the switch wired incorrectly. Based on your description, you should have:

Hot (black) from receptacle to switch.
Neutral (white) from receptacle wire nutted to white going to the light fixture.
Switched hot (black) from switch to the light fixture.

If the switch is a smart switch, dimmer, motion detector, etc. then the white will also go to the switch.
If you still can't figure it out, upload pictures of the wires going to the receptacle, to the switch and to the light fixture(s).