If you have time to download and install just one of the apps on this list, make it Avast! Security and AntiVirus. Avast has been in the online security business for over a decade, and is a recognized leader in the antivirus and backup space.
Avast's toolset is remarkably comprehensive - even with the free version. Installed on your mobile device, it will automatically detect and delete viruses, block sms spam, and even block telemarketing calls. The premium version includes advanced data backup and anti-theft features.