+118735 
+118735 Please take a look at this unit of work.
http://www.mathsisfun.com/geometry/unit-circle.html
I do not like their explanation of tan much but their explanation of sine and cosine looks really good.
Try and understand the interactive pics properly.
Basically if you draw a unit circle (a circle of radius 1 unit) centred at (0,0) then
$$\\sin\theta = y \;value\\
cos\theta = x \;value\\
tan\theta= \frac{y}{x}=\frac{sin\theta}{cos\theta}$$
The (x,y) value that I am referring to is where the arm of the angle cuts the diametre of the unit circle.
The diagrams in the unit are trying to show you why this is so.
The more you understand the less you have to memorise and the easier it is to move on to the next step. 
