Ella -
You can always find the y-intercept by setting x equal to zero and solving for y. For a quadratic function where y=ax^2 +bx +c, the y-intercept will always be equal to c.
But understand that sometimes c might equal zero in which case the y-intercept is simply zero. A good example is y=x^2. Try graphing it and see where the y-intercept is.