Start with your lowest zoom eyepiece and work your way up from there. There is no "best" eyepiece really, if you feel like looking at a zoomed in blur, then that is fine! The further you zoom in the blurrier it gets. Also try to get saturn at it's highest point in the sky for best viewing. Later on in the night as it gets lower you will have more distortion from the atmosphere.
Your telescope is about 4.7 inches correct? (120 mm across) You won't be able to get a large crisp image with that unfortunantly. (At least in my meager experience)
Also make sure your scope is collimated correctly.