For versus To

Giving

• She gave it to me.
• I donated to Farm Sanctuary.

Direction of communication

• She talked to me.
• I said it to him.

Destination

• I'm travelling to Lima.
• I'm going to the store.

Distance

• It’s 20 minutes from my job to my house.
• It's 3 hours to Washington.

Use For in the following situations:

Benefits

• Exercising is great for your health.
• I eat a lot of vegetables for the fiber.

Purpose (with -ing verb)

• A helmet is used for protecting your head.
• This tool is used for cutting metal.

Assisting someone

• Could you carry these books for me?
• I can cook for you if you're tired.

Length of time

• We’ve lived here for 2 years.
• I can cook for you if you're tired.

When referring to motive/purpose/reason it get's a little complicated.

If you follow the preposition with a noun you use for.

• I'm going out for food.
• She's working hard for that possible promotion.

If you follow the preposition with a verb you use to.

• I'm going out to eat.
• I volunteer a lot to help others.