What transistor should I use? And how do I determine that?
As already mentioned in the comments there is rather important information missing to give an accurate answer to the "what". But here are some steps you can take to come up with a possible solution yourself and then come back with a more specific question like "will transistor xyz fit my application".


Figure out what kind of transistor you want to use in term of NPN or
PNP (maybe even a Mosfet?) 
Determine the current your transistor has
to be capable to draw
Determine to voltage the transistor has to withstand
Define the maximum power loss you can have in your transistor,
consider switching losses if you have a fast switching application.
Open the search mask of a big distributor like mouser and try to find a transistor that meets the given criteria.
Check the datasheet for other parameters that might influence your design, if you need further help just come back with specific questions.