Rub it really good with a little olive oil on a paper towel. Don't put it in the dishwasher; it'll make it worse.

It may not cook good once it has rusted. You will probably need to 'season' it again at least. http://www.melindalee.com/Cast-Iron.html

This is what I do. http://testkitchensecrets.cookinglight.com/tks/2009/01/how-to-season-a.html +5

1. Take a potato and cut the end of it off. This will be your scouring pad. Put Ajax or Comet Cleanser in the rusty cast iron cookware. Alternatively you can soak the pan in vinegar causing a chemical reaction that causes the rust to decompose. Use the potato to rub the cleanser around. You will notice that the end of the potato changes and gets "slicker". When it does this, slice a thin slice off the end of the potato that you have been using. 2. Rinse the skillet and put a bit more cleanser in the pan and scrub with the potato again. 3. Repeat the above process until the pan has all the rust gone. 4. Wash with liquid dish soap and water and rinse well. Then dry. 5. You can leave it like this if you are just going to deep fry in it. But if you don't want it to stick, do the following steps. 6. Put a bit of oil or shortening about 1/4 in. deep (5 ml) in the pot and rub onto the sides up to the top. 7. Put into an oven at a low temperature, say under 300 ºF (150 ºC) and cook the oil or shortening for several hours. 8. In the future when you use the pot or pan, if it sticks, repeat step 7 until it stops sticking.

American Test Kitchen recommends re-seasoning.

https://www.thekitchn.com/how-to-restore-a-rusty-cast-iron-skillet-cleaning-lessons-from-the-kitchn-203086