Downloading a file, any kind of file, that has one attached to it. This includes from websites, email attachments, etc.
Downloading an app that has one in it or attached to it.
Downloading an app that allows for other things to be downloaded and installed on the device without your knowing it.
Connecting the device to any other device that has one living on it which can then transfer itself over to the tablet.
Connecting the tablet to a device to get files from that device, which can be infected.
Installing a SD card in the device that has infected files of any kind.
Rooting the tablet and the new OS you install is infected.
There are more ways, but those are the more common.