While deciding to buy a bicycle, you need to consider few parameters such as age, gender, price, quality, the purpose of buying, type of bicycle/features etc. For ex: if you are a boy of age 24, then you would consider to buy a bicycle with height of 18" inch, with gear & shocker, etc.
And if you are a girl of age 6 yrs, then you would consider a kid friendly bicycle with good safety features that the kid can ride.
Budget, quality and the person who will ride the bicycle are key thing. Start with exploring the options based on budget and the type of bicycle that you may need and then narrow down the bicycle basis your requirement. And then compare with features, price and quality that can help to narrow down the selection process.