I have Courant's Introduction to Calculus and Analysis and also have Spivak's Calculu's and Thommas-Finney Calculus with analytic Geometry, I'm finishing my precalculus studies, and I don't have much experience with calculus.
I want to become really good at math, and I want to know (Real) calculus, so I'd like to know if Courant is good for a first approach or should I start with Spivak?
Courant and Spivak are classic excellent texts, from different eras. I suggest you use both, in parallel. You will learn a lot comparing the two approaches.
A word of warning: both these books work at a level of rigor that you may not be familiar with from your precalculus studies. Thomas will have a larger collection of relatively routine exercises.
Whatever route you choose, be sure to work lots of exercises.