How to approach a two weeks notice?

So I've only been at my job for about two weeks and is planning to resign due to that a little family incident happened and I wont be able to get a ride to work anymore. I asked them if it was possible for me to switch shift since I just started but according to their policy, I dont think I'll be able to switch. With that in mind, I really still want to work there, so I just dont want to quit without notice incase in the future, I might want to come back. I wanna give them my two weeks notice. Do I just tell them or do I actually have to write them a letter saying that I might apply again in the future?