Depending on the current in the 12V circuit, the switching speed required and the level of isolation you need, and possibly some other things, you could try a mechanical relay (which might itself need a transistor to drive enough coil current), an optoisolator (there are many types, from low power ones to ones that can switch back-to-back power mosfets on and off to control bidirectional currents), or for some very specific sorts of isolation, a transistor circuit. Another possibility might be to drive a high frequency square wave out of the microcontroller, through a transformer, to a rectifier, to the gate of a power mosfet. But not all of those ideas would work for every possible situation that fits your very short description. Tell us more.
The key word here is 'isolated'. Depending on how much current you need on the 12 volt side, the usual devices are optocouplers( for lower currents) and relays (for higher currents).
You can drive optocouples with your 3.3 volts. You may have trouble finding relays that work that low. In that case, you need a transistor and extra supply voltage between your I/O pin and the relay coil.
Actualy What i want to do is use the outputs to drive high current relays and i figured 12v is a common voltage for relays. the isolation comes in to play because i wanted to use a seperate source to drive the relays but i could avoid this with the use of a switching power suply; on the inverse i will need to use the same 12v source to switch the
3.3v on the inputs of the microcontroler but this could be done easily enough with lots of options;;;; im just not sure about the outputs