there is nothing to "do" here ... this is just a matter of knowing the unit circle.

note that

here is how to find a trig value for an angle involving that is not on the unit circle ...

you need the difference identity for cosine to calculate the exact value ...

If none of this makes sense to you, then it's obvious that you have some major gaps in your knowledge of basic trigonometry. you should see your teacher to get some face to face help.

finally ... "cos(0pi/12)" = cos(0) = 1 , not 0.