Clothes (I would hope you do...). Typical semi-formal wedding clothing will be perfect.
There will be an open bar for you to enjoy for one hour during the cocktail hour, as well as one hour after dinner. The remainder of the night will be at your own cost, so get your drinks in while you can!
When you enter your name on the RSVP page, you will know if a plus one is included for you.
The ceremony is scheduled to begin promptly at 5:30pm, so anytime before then is appropriate.