Answer:
The maid of honor's job is to be the bride's best friend. As her closest friend, you should continue doing all the things that a best friend has always done. Be there for her when she needs you, listen if she has problems, help her if you can, get excited with her over what is going on in her life, etc.
Typically, a MOH chooses to throw a shower or bachelorette party, and she will typically choose to help the bride with a few wedding tasks. While not required duties, these are nice things that you should do for your best friend if you can.
You will probably also need to sign the marriage license, and you can give a toast or a speech at the reception if you want to.