For versus To

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.
She worked there for 5 months.

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.

Use To in the following situations:

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.