Our 4 year old yellow lab has areally red and itchy ear what is causing this and how do we cure it?

Sounds like it's a yeast infection. Take her to the vet - they will take a swab to make sure that's what it is, and perscribe proper meds based on what they see (different things are given based on if there are blood cells in the ear as opposed to just yeast, etc)

Sariss is right.

The most likely causes are infections, hair growing inside the ear canal or mites.

You can only get the medication from your vets. You will most likely be given ear drops that are to be used several times a day.

When you use them hold your dogs ear back over the top of its head and put the required amount down the ear canal.
Flip the dogs ear back over and rub. The rubbing pushes the medicine further down the ear and makes it more effective.

If you have a difficult dog then I use a syringe (without the needle) and draw the required amount of drops, this way you can just squirt without having to worry about counting out the right amount of drops.