I want to play as a trickster but need to get the build right, help please

Definitely invest in Illusion. I wouldn't set Illusion as a Major personally (it levels way too fast) but that's good school to practice.

While your character is low-leveled, get him or her Sneaking a lot, too. Get good at sneak-attacks (I am assuming you will attack other NPCs, forgive me if that's wrong).

The Shadow starsign is good because it allows you to go Invisible, which can come in handy at lower levels, too. Other than that, I'm a little confused.

Why is this character going to be a "trickster? and. How would you define "Trickster?"

Someone who relys on magic to go undetected and who uses sneak attacks to kill with ease and someone good at getting on npc's good side this is my idea of a trickster