Cast iron skillet generally develops rusty surface if you happened to cool it down by running under tap water. Cast iron should be allowed to cool slowly. It may also be the case that you didn't use any oil in the beginning.
You can wash the skilled and do the seasoning again. I follow below steps to season my cast iron skillets which are used for making thin rice crops (called as Neerdosa in Karnataka, India)
Pour in dry salt and slowly heat the skillet. Make sure you have covered the whole cooking surface with salt. Let the salt remain for a while till it starts getting a pale yellow/brownish colour. Add a small amount of water and this makes a white salt coating on your skillet. Turn off the heat and let it gradually cool down.
Brush oil all over the pan and turn on the heat. Once the oil starts smoking, turn off the heat and then keep it for cooling.
This is a good starting point. You can now start cooking and remember to oil or water whenever you are cooking. Keep coating the skillet with cooking oil once in a while to avoid oxidation and formation of rust.