Driving a 12V door strike from 3V3 BeagleBone?
You'll have to connect the lock on the collector side of the transistor; common collector like you've drawn doesn't work.

  

The BC337-40 suggested by Russell is a good choice; it has a high \$H_{FE}\$, and can draw up to 500 mA.  

Your power supply needs some attention. You need 12 V at 250 mA, that's 3 W. If you have a boost converter with 85 % efficiency this will need more than 1 A from the BB, and that's probably more than it can supply. Definitely too much if you're powering the board from USB. I would power the lock from a 12 V/1 A wall wart.