You'll need a BTTV subscription otherwise the only way will be to stream from the internet, with the device connected to your telly if you want.
The only way of decrypting the BT Sport 1&2 channels on DTT 58 & 59 is using a BT Vision+ box and viewing card. That requires a subscription to the BT TV service.
Also from 25th Jan 2014 BT have stopped providing this viewing card service to new BT TV subscribers. If you want to watch BT Sport on a BT TV box (either BT Vision+ box or BT Youview box) with a BT TV subscription you now need Infinity broadband. The other options are the Sky satellite and Virgin cable delivery methods.