What has worked for me when restoring cast iron is to take some steel wool and polish all the rust off the skillet. Then wash/scrub it until you've got a clean surface. You may have to repeat the steel wool step and wash several times. Don't let it sit around wet for any time at all (I swear they can rust in minutes). I don't like to use
soap on cast iron but you can use a little mild dish
soap as this point to make sure you've got all the rust dust off, especially if the pot is sticky from old grease. Then dry it right away. Now you want to season it. This involves coating it with some kind of fat, oil, etc.--something edible. I like lard the best. I also find that industrial vegetable oils tend to create a sticky surface, but your mileage may vary. Just use a little bit, you don't want a sticky surface. Rub it all over the skillet inside and out until it's evenly coated. Now you want to heat it in the oven for about an hour. I do it at around 350F. If you turn the skillet upside down some of the grease may
drip, which is good, but you want to put a pan in the bottom of the oven to catch the drips. Let it cool until it won't burn you. Now you may want to kind of polish it with a clean rag to remove any excess oil, especially if it is sticky to the touch. Now it's ready to cook. You want to add more grease than you usually would when you cook for the first few times, so your food doesn't stick and to build up some more seasoning. Just cooking in it with plenty of fat is often
enough to maintain a cast iron skillet in good condition, but after one has been restored it's a good idea to go through the seasoning process (coat in oil and heat) a few times to get it in good shape. And if it ever starts getting food stuck to it or just looks kind of dry and sad, season it. If you use a flat metal spatula to cook with over time it will smooth out the inside even more and less sticking of food will occur.
If you cannot get all the rust off with steel wool you can take it to a machine shop and have it sandblasted back to the original cast iron (which is silvery, not black) and then go through the seasoning process (probably multiple times) until it is a nice, glowing black.
Ebay is a good place to find cast iron. Griswold is a good brand.
Edited to add: Never put hot cast iron in cold water, it can crack. After the initial cleaning, it's best not to use soap unless you have to. Many
people don't even use water, depending on what they cook, but just wipe it out with a cloth, especially once it is well seasoned and nothing sticks. If you do wash it with soap it's best to give it a seasoning afterward. If you just wash it with water, dry it out right away and maybe warm it gently on a burner to make sure it's really dry. Also don't scrub it with steel wool ever again unless you have to, since that can scrub away your seasoning layer.
Edited again to add: I see you say you don't have a self-cleaning oven, but I assumed you have an oven of some kind. If you have no oven at all it can be seasoned on the stovetop, a grill, or even over an open fire (with some finesse). Be more careful as these options are hotter than the oven. If grease starts to smoke let it cool down, and don't leave it the full hour. If it gets too hot while empty it can crack or ruin your hard work getting the surface back in good shape.
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