Where to start?

Where to you begin? you've been away from the bicycle seat for such a long time where do you go for
advice? Well a good start is a proper, established, only do bikes, bike shop. Do not be tempted to take the cheapest option either through a catalogue or a supermarket. Your new to you bike needs to be set up for you so that you feel comfortable and balanced, this will only occur if you get the right advice and adjustments. Within the cycle trade the kind of cheap bikes that are sold through high volume retailers are known disparagingly as BSO's (bike shaped objects) this is because they are not built to be maintained and in many cases the materials used in their manufacture will deteriorate so quickly in our climate that they become unsafe and unusable very quickly.

What type of bike to ask for? well if you're intent on going regularly off road and up and down a few mountains then probably a mountain bike would be best. However if you just want to get from A to B on a nice smooth road that has a few hills then ask for a relaxed tourer. This bike will have an upright riding position, fairly narrow tyres and only about 5 gears, it will also have the facility for carrying loads i.e. a rack on the back or front. Now this bike might not have the "wow" factor but set aside your desire to look sexy and instead consider the practicalities of your purchase. Sorry it's boring advice but save the mountain bike and the head down racer for later on when you've become so enthused by the whole cycling experience you sell the car and have a garage full of bikes instead.

If you can afford it, definitely consider hub gears as opposed to the more common derailleur type, they are
easy to use ,more reliable, need less adjustment and generally a less messy option.

If you can find a good second hand bike then be prepared to have it checked over thoroughly before you buy it. If it has been regularly maintained then it will be as good a buy as a new one for reliability, but if it's been neglected you might spend as much in parts to recondition it as the original cost of the bike which will probably make it pretty close to the price of a new machine.

Once you've found your bicycle don't forget to gear yourself up too. Helmets are not a legal requirement but they are a good idea for many reasons not just the obvious one of protecting your nut in a fall. They can aid your visibility , particularly when it's raining and the helmet has a peak. They can help you be seen by other road users, many of them have inset LED lights and fluorescent decals. They keep your head cool in the summer and warmer in the winter, baseball caps and bobble hats just make your head sweat. They can take the brunt of a flying insect direct hit, surprisingly painful in the wrong place.
Get two pairs of gloves , a full fingered variety for colder days and a fingerless pair for the rest of the year, you'll avoid sore and sweaty palms if you use gloves. If you don't wear spectacles consider a pair of cycling glasses, particularly if you wear contact lenses normally, as the roads are a dusty environment.
Light weight breathable waterproofs are a really good idea for those unforeseen downpours.
For the rest of your riding clothing wear a few removable layers rather than one thick layer, it's easier to keep a comfortable body temperature. Avoid baggy trousers and wear shoes with a good grip. Most importantly make sure that you are visible, if your clothing isn't going to do it then make sure you wear a fluorescent hi-vis vest

If you're going to use the bike regularly then it's worthwhile investing in some good quality load carrying equipment. Panniers are the ideal and safest way to carry your shopping back, there are lots of different options but the easiest combination to ride with are ones over the rear wheel. You can get some excellent trailers for bikes now, it's worth paying a bit more for ones with high quality fittings and a bike hook up that fits somewhere near the rear axle. Do not under any circumstances be tempted to hang shopping from the handlebars, it might seem safe enough when you're stood still but the swinging mass will create problems once you start pedalling.

Ok so now you've got your bike and your gear anything else you need to know?... how about some instruction on how to stay safe on the road?  take a look at Cycling tuition