Chemistry help with ions please?

Hi, basic concept but i just need some clarification please. So when an atom gains or loses an electron it becomes an ion. If it gains an electron to receive a full outer shell it will become a negative ion and the atom it gained the electron from will be a positive ion - (would this now be an ionic compound? ), if the ion that gained an electron - the negative ion, is oxidised, then is it a random electron on the outer shell that is lost or the electron that it gained from the other atom?
I may have this really wrong but yeah... any help is greatly appreciated! Thanks