The most important tip: just ride. There will always be an excuse-- (it's too far, my bike is too old, I'm not in shape, I don't have time)-- but if you want to start riding again, then get on a bike and go! These tips are the basics to keep you safe and help you enjoy the freedom of riding on two wheels.
Learn basic riding skills. This is essential before you go riding in traffic. Find somewhere nearby where you can ride without worrying about other vehicles so you can master the essentials – parks, trails, and quiet car parks or super quiet streets are ideal.
Stopping – make sure you can stop quickly and safely if you need to.
Turning – practice turning quickly and confidently to avoid objects or make a turn while riding at speed.
Observation – practice looking over your shoulder and to your right and left while riding.
Riding with one hand – lift one hand at a time off the handlebars and repeat until you can control your bike with either hand (this is so you can learn to signal turns with your hands)
Get to know your gears. Unless you're lucky enough to live somewhere that's pancake flat, being able to use your gears effectively will help you get from A to B without too much huffing and puffing. Before you start mixing it with traffic, get to know your bike's gears and what works best for your legs: try climbing, descending, and cruising on a flat section. Are you a spinner or a slow turner? Everyone has his or her ideal rhythm (or cadence) so take the time to find out what suits you.
Consider a helmet. Helmet laws vary by state, but are recommended. While helmets can seem a little cumbersome or uncool, they help protect your head in case of the unlikely event of an accident. When putting yours on, make sure it fits properly. It should sit horizontally on your head and shouldn't move about. Two fingers should fit tightly between your chin and the strap.
Watch out for the 'door-zone'. Ride at least 3 feet away from parked cars. This way you'll avoid any unexpected door openings!