More than most sites in Athens, the agora requires some kind of guide: person, book, or audio. If you don’t know what you’re looking at, if you can’t listen to the stories and understand the history, you won’t get much from a visit here.
With a guide, you should plan on spending at least an hour at the agora. I’ve spent half a day here and enjoyed every minute. The more you know (or, at least, the more your guide knows), the more enjoyable your time here will be.
You will be asked to check larger rucksacks and bags either at the ticket booth or (more likely) at the Stoa of Attalos.
Wear comfortable shoes with firm ankle support. Much of your walking will be “off trail” on this site, so watch your step carefully. It would be easy to turn an ankle or take a tumble here.
Water fountain and restrooms are located in the Stoa of Attalos—on the north end.
Videos and photos are allowed throughout the site. Because of the poor state of preservation of many of the buildings here, good lighting will be crucial to good pictures. Try to come in the morning or evening to get shadow lines and some contrast in your photos.