Along with every other aspect of weddings, invitation ettiquette is ever evolving. For this reason, I will not delve too deeply into the subject; however, there are a few key things that should be considered. For more detailed information, browse any popular wedding site, such as: The Knot or Wedding Channel.
For many people, weddings by nature are significant, formal events. Therefore, names of adults should have appropriate titles: Mr., Mrs., Ms., Dr., etc.
Children over the age of 18 still living with their parents or guardians should be sent their own invitation. Children under age 18 may be addressed as "and family". For example, "Mr. and Mrs. Jones and family".
Street names should be fully spelled out, rather than abbreviated:
St --> Street
If you choose to include an RSVP envelope, it goes inside the main envelope with the invitation. It can be calligraphed, handwritten, or addressed with a pre-made sticker.
All that being said, it's still completely up to you. I will write it however you'd like, to suit your style and the theme of your event.