Given that a point is marked on the unit circle say it is

That meanns base of the triangle

and hight of the right angle triangle

Since it is unit circle so automatically AC=1
or you can use Pythagorean theorem to find side AC.
Now we can use ratios of sine, cos and tan to find their values as follows:



Same way you can find values of sin, cos, tan etc for any given point on the circle.