Question 27825
Unfortunately the answer to the question "I don't understand hyperbolas" can't be answered with a quick answer. You'll probably have to read the chapter in your book a time or two. For some additional information go to: <a href="http://www.algebra.com/algebra/about/history/Hyperbola.wikipedia">Hyperbola.Wikipedia</a>
Anyway, finding the foci involves finding a, b & c (that's what they're usually called, your book may vary).  The equation for a hyperbola is of the format:

{{{(y-h)^2/a^2 - (x-k)^2/b^2 =1}}} When major axis is y or:
{{{(x-k)^2/a^2 - (y-h)^2/b^2 =1}}} When major axis is x
In either case the positive term is the major axis, and determines where the foci lie. Frequently h & k will be 0. In any case c is defined by:
{{{c^2=a^2+b^2}}} (contrast with the definition of c in an elipse {{{c^2=a^2-b^2}}})
Which reduces to {{{c=sqrt(a^2+b^2)}}}
The focal points when the major axis is y will be: (k+c, h) and (k-c, h)
and when the major axis is x: (k, h+c) and (k, h-c)

For your problem:
{{{y^2/36-x^2/4=1}}}
a = 6 (square root of 36)
b = 2 (square root of 4)
c ={{{sqrt(36+4)=sqrt(40)=2sqrt(10)}}} approximately 6.325

So your foci are at (0, 6.325) and (0, -6.325)
Confused yet? That'll happen. Do some practice problems, read the book, and ask some specific questions if you get stuck again. Also try http://mathworld.wolfram.com/Focus.html for a better explanation of the foci for elipses & hyperbolas.
{{{graph (300, 300, -10, 10, -20, 20, sqrt(9x^2+36), -sqrt(9x^2+36))}}}