CORRECT: "It's good experience...practice for when you get married one day." Experience is a non-count noun (and so is practice) so it's not necessary to use an article here. However, you can say, "a" good experience if you are talking about a particular experience. Without the article, it's advice about a general experience. ( The difference in meaning is slight.) INCORRECT: It's good experience...practice for you getting married one day. CORRECT though not common: It's good experience...practice for YOUR getting married one day. Or you can say, "It's good practice for marriage/married life." Depending on what "it's" is referring to, this could be the best answer because "getting married" could refer to the ceremony or wedding day rather than the marriage itself.

"It's a good experience... It's practice for when you get married one day." You need to put "It's" in both and "a" before good. If you have any questions, feel free to ask. :)