diaper rash

My 6 day old has a diaper rash. I'm a FTM so I don't know much. And not sure how he got it. I only noticed it this morning when my boyfriend said something when he changed him. So he's had two Changings after that and I've been putting cream on him. And I'm sure a little vasaline gets on him because he's circumcised. How long will it take to go away? How do I know what caused it and help to prevent it? Idk if it really bothers him or not he acts the same as he did before but I still feel really bad🙁