A question about digression

if I want to write about how my world has shaped my dream of learning computer science, is it ok to write about something unrelated to computers to prove my leadership and creative mind? like organizing a club or making plans or something else...

It's absurd to write unrelated things on your essay.
However, as these are quite general skills, you can just blend them into your essay. Mentioning in 'the route to reach my dream', maybe?
Of course, don't write about it too much. Concentrating on the subject all along the way is crucial. Let readers be inspired by your passion and impressed by your experience.

but if I don't have so much experience about computer science to demonstrate my creative mind or leadership?

Making things up, maybe? I think it's not really harmful to include some of "your" experience of leadership
For me, a creative mind can be demonstrated through the new ideas you put in your essay

ok, maybe I can change another theme...

How can i write about the experience of leadership without saying that it is me to organize the activity or something like that?
Praising myself in the essay is not permitted, right?