I wish to drive a MOSFET using a 555. I dont have a driver handy, will it be fine to use a transistor instead? So the output of a 555, could activate a transistor(as swtich), which allows a source to drive the FET?
The 555 output can Source or Sink 200 mA so unless you want to drive the mosfet in high frequencies it will be more than enough. A single transistor is probably going to drive the mosfet worse than the 555 itself
If you are need to use a driver between the mosfet and 555 then you are better off with a push-pull (totem pole) driver configuration that can sink/source current equally strong in order to turn on/off the mosfet quickly (again only if needed)
or even better with a dedicated mosfet driver like TC4426/27/28
You could do it this way:
Just hook up the outgoing end of R2 (where it says "To Arduino") to your 555 output. Note that this circuit is inverted - the load will be ON when the 555 input is LOW. If that is a problem, just put a inverter (74HC04 or similar) in between your 555 output and R2.
If you have what's known as a "logic level MOSFET", you don't need the NPN transistor because logic level MOSFETS can be switched by a lower voltage such as the one provided by the 555. Usually these will have the letter "L" in the component name (for example IRL540). If it's a logic level MOSFET, it should clearly say that in the datasheet.
If it's not a logic level MOSFET, then you need to use another component to switch a higher voltage (12V in this case). NPN transistors work just fine for this purpose. Regular MOSFETs typically need close to 10-12V to switch on (notice that MOSFETs are voltage-driven and not current-driven).
If you'd like to read more about driving MOSFETs, I'd recomming the following reading: