This seems like a pretty silly statement, but the first step in picking the right smartphone is to make sure you actually need one. Smartphones are expensive, and so are the coverage plans they require. It's easy to use more data than your plan allows and wind up with expensive overage charges. Even if you don't go over, you could end up paying for a data plan that you barely use.
So, how do you know if you need a smartphone? Look at how you use your current cell phone. Maybe you only use your phone for talking and the occasional text message. If you already have a GPS system and an MP3 player, you can make do with a regular phone that doesn't offer music or directions. You might not want e-mail on your phone. Getting a smartphone often means having immediate access to work e-mails, social media and the Web. Not everyone wants to be that connected.
On the other hand, think about how you'd like to use your phone. If you hate feeling like you're missing out on the latest tweets, updates from your friends, and news, then a smartphone will probably seem worth it to you. If your job requires you to be connected all the time, providing immediate responses to e-mails and questions, a smartphone makes sense. You may already have an MP3 player and GPS, but having all those features in one handy device can make life easier and less cluttered. Finally, if your lifestyle has a long commute on public transportation, or if you often have a lot of downtime when you're in public, a smartphone can help you pass the time.