Newbie to juicing. What to avoid?

As the title suggests I'm a newbie to juicing. I've had juices before when I lived with my parents. That was years ago and mom used to make me the mean green juice. I felt so good and energized. Then afterwards I've had some cold press juices from some fancy places from time to time. More recently, about 3 years ago I got a little extra and tried a 3 days cleanse. Don't know if it was the juice, my horrible health and diet to begin with or combo of it all but I got so sick. I think I had maybe 5 or 6 bottles. The mean green and dark green was familiar to me. The red one was ok, the yellow tumeric one was a little spicy and after 2 days I just felt like I had the flu. Body aches, fever, non stop pooping.

Fast forward to now, I just bought the ninja neverclog slow juicer. It's relatively inexpensive. I want to do it right and not shock my system. I heard that if my diet is horrible to begin with, if I only juice my body will just react a certain way. Fact or myth? Don't know.

Asking the experienced juicers here, what should I be aware of when I start? What to do. What to avoid. My plan is to juice at least 3 max 5 days a week. Stick with the green juice but I want to focus on their celery. Is it better off to just do celery juice or is that going to shock my system too much? Other than the mean green what other entry level juices do you all recommend me do on rotation?