The first thing that minimizes the variety of models is the price. We divide all phones into three price brackets: budget phones that cost around $200, mid-range phones with prices from $300 to $550 and expensive brand new phones that cost more than $750. Decide how much you can spend and lots of models will be immediately out of your consideration.
The second thing that also narrows your choice is the operating system. It is not possible to tell which of them is better; it is all about personal preference. You can choose from iOS, Android, RIM and Windows. iPhone runs iOS and Blackberry runs RIM. So if you prefer one of them you know what phone to buy! But you can hardly find those phones among the budget models. If you prefer Windows or Android you have a greater choice. Just look for the latest versions of the systems since they are more stable and secure. If it is Windows, look for 7.5 or 8. If it is Android, then 4.0, 4.1 or 4.2 is what you need.