Every Wedding Reception requires someone to officially administer the program. Sometimes that individual is a designated Wedding MC - a family member or a friend - and sometimes it is the Music DJ etc. The one thing they both need to do is to move the Wedding Reception Program on smoothly from speech to speech, from cutting the cake to the first dance.
Every Reception is planned differently depending on customs, religion, family backgrounds etc. If you are asked to be the Wedding MC, chances are the program will be fairly well planned by the bride and groom and individuals in their family. You might need to remind them of possible program events they forgot about, like a Welcome to the Family Speech or an announcement about a Gift Opening the next day. If you are the Wedding MC and there is no conventional program, try to get the bride and groom together and decide on one. It will make your job much easier.
Once you know the program, the order of events and the names of individulas you will be introducing, your job is half finished. The best way to proceed through a wedding program is with appropriate and tasteful wedding jokes and humor. A funny Wedding MC can make the difference between a so-so Wedding Reception and a Great one.
Don't worry or put too much pressure on yourself. Just plan to use humor when you get up to speak, or tell a tasteful wedding or marital couple joke as you introduce someone to make a toast or speech. If you use humor right away the audience will expect you to make a joke or two when you get up to speak and this expectation actually makes it easier to be funny.
The key to successfully using humor is to personalize the joke. The Wedding MC Jokebook goes into this technique in detail if you are concerned.
Remember, as a Wedding MC, your job is to move the Reception Program along smoothly from speech to speech or event to event, and to use humor when doing so. If you can attain that you will do a fine job as Wedding MC.