- “C'est pour toi ” means “it's for you.”
- “C'est à toi ” means¹ “it's yours”, or¹ sometimes “it's your turn” (to play, or to do anything else).
This thread is of interest, and seems to suggest you're trying to translate the english “to”, and has many links expanding on “basically it's a huge question”. That gives:
À is a very important French preposition. Its many different meanings and uses in French include all of the following:
- Location or destination
- Manner, style, or characteristic
- Purpose or use
The French preposition pour usually means “for,” but has a few other possible meanings as well. It can be followed by a noun, pronoun, or infinitive, and can be used to indicate all of the following:
- Purpose / Intention
- In favor of
- Point of view
- In place of / In exchange for
1 : Arguably, that's still the turn that's yours, so whe're still talking about possession, but oh, well.