i- adjectives ans na-adjectives in japanese

ok im slightly confused as to which of these to use when describing something.
the examples i have seen show
aka-i (red)
kirei-na (pretty)

taka-i yama (a high mountain)
kirei-na hana (a pretty flower)

Im confised as to which i use at which situations.

please someone explain this to me.

WHY ARE YOU MAKING A THREAD ON THIS YOU FUCKING WEEABOO GO TO THE JAP ASK QUESTIONS THREAD