Decide what you will use the bike for?
If you can describe what you want to do with your new bike, you'll make things much easier for yourself and the salesperson who's helping you choose. Different bikes are intended for different types of riding and some are very specific to their purpose. However, most bikes are pretty versatile, you can ride to work, to the shops and round the hills on the same bike. Many bike commuters prefer the more upright position provided by a flat handlebar, though. They go for hybrids bikes with the same large, fast-rolling wheels as the racing bikes but with bars and controls more like those found on mountain bikes. Hybrids and drop-bar commuter bikes are also great for exploring country lanes and bike paths at the weekend.
If your goals are a bit sportier, but still on tarmac, then you should be looking at road racing bikes, or sportive bikes. Both have drop handlebars for a variety of hand positions and aerodynamics, but a sportive bike will have a more upright riding position and usually a wider range of gears. If you want to head out onto the trails, then a mountain bike is the way to go. Perhaps you're already into outdoor sports and you want to get deeper into the countryside, or you fancy zooming round your local trail centre or bikepark. Either way, you need an upright, wide position for control, big tyres with lots of grip, a big range of gears, and powerful brakes.
Not confident of your fitness or want a little help getting up the hills? Take a look at electric bikes. The latest advances in battery and motor design mean that electric bikes offer a genuine advantage especially when it comes to easing hills and zipping away from the lights. If you're planning 'mixed-mode' trips, like a commute that involves a train ride, then a folding bike may be perfect for you, nothing beats their convenience or ease of parking. You can't sneak a full-size bike on a rush hour train or under your desk.Most decent quality folding bikes are almost as quick and as comfortable as regular bikes.
If you're not going very far, and you live somewhere flat, the classic English sit up style would be ideal, some models varying from single speed, 3 speed all the way up to 27 speed depending again on where you plan to ride. Younger riders will need specific kids' bikes. It's worth buying quality if you possibly can. Cheap kids bikes tend to be extraordinarily heavy and very poorly made, whereas if you spend a bit more your child gets a lighter bike that holds its resale value when he or she outgrows it.