"We sell stringed instruments, e.g. violin and cello." "This program focuses on developing your language skills, i.e. speaking, listening, reading and writing."
You should use e.g. when you want to provide an example and use i.e. to explain or rephrase something you have just said or written. If you're not sure which one to use, just replace the Latin abbreviations with their English equivalents and see if they're appropriate.