Take someone's side

Side with someone / Back someone up / Stand up for someone / Take up the cudgels for


Prendere le parti di qualcuno

Dare manforte a qualcuno

Meanings
To support someone and not someone else
Examples
The boy's mother will naturally take his part
La madre naturalmente prenderà le parti del ragazzo
Don't worry. We will all side with you on this condo dispute
Non preoccuparti. In questa disputa col condominio prenderemo tutti le tue parti