First of all, welcome to the forum 
Hm, not sure it's an easy answer what's the best trigger for Shepard to become good/bad.
I guess the main thing you have to do for what you plan is to flesh out your Shepard some more. Have you already settled with the background? From there you can spin a nice story around your Shepard, and then plan out how he/she would act for most situations.
When I ran my first FemShep a couple of months ago I actually wanted her to be a "test" character. I wanted to try lots of things I hadn't done before, which first and foremost included her being a mission first type, and being suspicious of aliens, including the crew. So she got the Colonist/Ruthless background because that fit my idea very well. She was a mix of Paragon and Renegade, so it was nothing I really set in stone, I just played her according to her mindset. She became trusting of her alien crew apart from Wrex, who she shot in the end (I used a "no persuasion" rule with her for most parts in the games). I enjoyed her run so much that she became my canon Shepard.
There are a lot of cool character ideas around, for example in this thread. You could give it a read if you maybe need some more ideas and to see how people approached their Sheps.
Another idea for my next character is a naive Paragon FemShep who believes in 2nd chances and the good in people/species, but she will reach a turning point in ME3 where she won't cure the genophage because Wreav will be in charge (I'll let Ashley shoot Wrex). She'll see that not everything can be solved peacefully and thus will change here a bit too (more towards Renegade). I guess there also won't be peace between the quarians and the geth 
Another member from here has a Paragon character who was above everyone else, a holier than thou FemShep that was a bit of an a*s to crew and other people. She changes after she saw that some shady characters (Kasumi the thief, Mordin the war criminal etc.) gave their lives during the Suicide Mission.
I guess... it really also depends on how far you're willing to go with killing off characters or doing mean things 
Anyway, I hope you can find some good ideas and have fun with creating your canon Shepard!