help (oneself) to
To take something for one's own use. (Whether one is actually allowed to take something, however, depends on the context.) Please, help yourself to anything in the cooler. I can't believe you helped yourself to the last piece of cake when you knew I wanted it!
help (oneself) to
1. To serve or provide oneself with: Help yourself to the cookies.
2. Informal To take (something) without asking permission: The thief even helped himself to the spare change in the jar.