find a website that offers hostings, you can go with freewebs who isn't affiliated with weddings or you could check out theknot.com or mywedding.com or even wedquarters.com
Step2
You want to choose a design that best fits you and your fiance` or maybe a color scheme that your using.
Step3
At the least you can include only when and where the wedding is, directions to the ceremony and reception site.
But if you really want to get into this, think about including you and your fiance`s biography, how you met, how you or your fiance popped the question. Have photos of you bridal party up with some information about each of them. Include attractions in the city the wedding will be in and lodging and travel information. Also have an online guestbook your friends and family can sign.
Last but not least, don't forget to include the gift registry information on your site. It's not a good idea to include this kind of thing in the wedding invitiation. However, theres nothing wrong with adding the link to your site in your wedding invitation.
Tips & Warnings
Do some research, find out which site works best for you.