riding a bike again after a long time?

Yes, get a real bike helmet, one with CPSC certification. Different types of helmet are certified (or designed, if there is no legal certifying agency) to protect against specific types of trauma depending on the activity. Bike helmets, for example, are designed to 'explode' on a single heavy blunt impact. On the other hand, my skydiving helmet is designed for repetitive smaller impacts*--it would not protect me in a bike crash.

The best way to start biking is to get on your bike with a proper helmet--and go. I started riding again a few years ago after 7+ years of being afraid to ride (following a nasty crash). I bought a bike and had no way to get it home except to ride it. So I did.

Get a pair of biking shorts too and keep your first few rides kind of short. Maybe 5-10 miles. There are muscles that will alert you to your new activity but they might wait a day or two. Short rides and biking shorts will make it easier to ride again within a couple of days of your last ride. And YES definitely get a bike helmet. There is no substitute for it and it may save your life.

What kind of bike do you have? If you will be leaning on the handlebars more, such as on a mtb or road bike, just make sure you have great cushioning under your palms, since they will be supporting most of your upper body. Hybrid or relaxed bikes allow you to sit upright.

Other than that...if you are riding in urban areas, ride with traffic and in bike lanes wherever possible. The reasons you ride with traffic and in bike lanes is because

1) The Cars' relative speed is lessened, increasing reaction time. Lets say you go 10 mph. If you ride toward a car going 40, that car's relative speed is 50 mph. But a car going in your direction will only be going 30 mph relative to you.

2) Cars pulling out of driveways pull through the sidewalk before looking or stopping more often than not. In addition, drivers don't usually look for pedestrians too much, let alone more distant bicyclists.

3) In combination with reason 2, drivers pulling out of driveways ALWAYS look left first, and if turning right, will rarely look right. If you are riding against the flow of traffic, you are approaching every car that pulls out from the right, thus increasing your chance of getting hit. If you must do this, ALWAYS make eye contact with the driver. Actually, ALWAYS make eye contact any time you may cross paths with a car.

Lastly, have fun! I have been an urban rider my entire life. I love riding. My legs love it too! :-)