Make sure the bike fits. A saddle too low or too high will make the ride much harder. If you haven't ridden, your butt will hurt but it will ease up with time. Just don't push too hard. Ride in a gear that is easy and give yourself plenty of time. Ride safe. Ride with traffic, not against it. Expect the unexpected. Carry a spare tube, patch kit and pump and know how to use them. Use lights. Flashing during the day. For lighting, for about $40 you can get this
http://www.dealextreme.com/details.dx/sku.9070
plus this
http://cgi.ebay.com/Bicycle-Bike-Omn...item45f6ed05db
plus this
http://cgi.ebay.com/Planet-Bike-Supe...item2c58ee4f9b
and do OK.