To Henry's excellent tips, I'd add a couple:
1)The unit circle begins at the ero point, at the extreme right (on the positive x-axis). Positive angles increase in the counterclockwise direction, negative angles go clockwise.
2) Coterminal angles are angles with different values, each of which lie at the same point on the unit circle. Since once around the is 360 degrees, Coterminal angles are usually 360 degrees apart, e.g. , 30 degrees and 390 degrees.
3) As Henry points out, the x and y side lengths are related in each of the four quadrants of the unit circle; they differ in SIGN. A reference angle refers to the difference between your angle and the nearest x-axis (positive or negative). The RA is always less than 90 degrees, and, by convention, positive. It allows you to find the absolute value of the relevant trig function (sin,cos or tan). The SIGN of the function is determine by a mnemonic ASTC (often = All Students Take Calculus). it tells us the sign of the functions in each quadrant:
A - Q1 - All functions are positive
S - Q2 - Only the sine is positive
T - Q3 - Only the tangent is positive
C - Q4 - Only the cosine is positive
4) Last, remember SOHCAHTOA, and that H in the Unit Circle is always = 1