"I'm 18 and my eyes prescription is -8 and it's not stable, so I want to do a LASIK surgery now, and another one later in life (around my mid-20s) in case my eyesight got worse."
Not a good idea. And not the right way to think about the problem:
is not like a magic wand that makes your vision normal. It is an effective way to compensate for being nearsighted (myopic)
by weakening and thinning the cornea. The more myopia, the more thinning and weakening of the cornea. And this doesn't go back to normal. Ever.
Your prescription is rather high right now. Having more LASIK later in the event of your becoming more myopic could easily cause serious damage to your cornea -even to the point of needing a corneal transplant.
Look into some other way to treat your high myopia, perhaps the ICL.
J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic