Any English business or grammar book will give you the format for a letter qne show you a sample. Your task is to write the body of the letter. My suggestions follow.
Make it simple. Tell them what you want and why.
"Dear XXX:
I would like to request permission to take [a week, 10 days, whatever] in order to get married. The wedding is on [date]. I would like to leave work on [date] and return on [date]."
If you have other helpful information, add that. Perhaps you have worked for them a long time and this is your first leave. Perhaps you have arranged for someone to fill in for you. Everyone knows that a wedding takes time to prepare for and that a honeymoon after is common. So you don't need to explain that part of it, unless there are special circumstances. Perhaps you have made plans to travel together after the wedding. You could say that.
Let me know if there is something else you think you should put in the letter.