Welcome to Android Central! You'll get a lot of differing opinions about this. It's still very rare to get an actual virus on your Android device, because you have to approve of the installation of that infected app--there aren't any "drive-by" viruses on Android (yet). If you install apps from well-established sources like Google Play or Amazon Appstore, the chances of installing an infected app is extremely low, since they both do a lot of vetting of those apps before allowing them to be installed. If, on the other hand, you install apps from less reputable sources (like downloading a free installer file of an app that is normally not free, from some filesharing site), then you will have a higher chance of getting some kind of virus.
You can still end up installing annoyingware from legitimate sources (e.g., apps that include popup ads on your homescreen), because that isn't considered to be malware per se. You can avoid this by reading the app's reviews carefully, looking for complaints of adware.
So many will argue that you don't need an antivirus app as long as you're careful about your sources. My view is that malware authors are probably some of the most ingenious people around (and it's sad that they don't use their intellect to make the world a little better), and that it's hard to predict the evolution of Android malware. So it isn't a bad idea to install a good security app, thereby outsourcing that concern to people who are experts in the field (like Lookout, Kaspersky, AVG, Avast, etc.). These security apps often have handy antitheft and recovery features as well--for example, with Lookout, if someone tries to unlock your phone unsuccessfully 3 times in a row, it will take a picture with the front facing camera and email it to you, showing you who is trying to unlock the phone.