There exist an infinite number of different parametric equations for a curve. The simplest way to get parametric equations for y= ax^2 is to use x itself as parameter: If x= t then y= at^2. That can, in fact, be done for any function- if y= f(x) then x= t, y= f(t) are parametric equations.
For non-function curves, we have to be a little more creative. For example, the relation, x^2+ y^2= a^2 describe a circle with center at (0, 0) and radius a. We know that cos^2(t)+ sin^2(t)= 1 so a^2 cos^2(t)+ a^2 sin^2(t)= a^2 so we can take x= a cos(t), y= a sin(t) as parametric equations. Of course, x= a sin(t), y= a cos(t) would work as well.
For a more general circle, (x- x_0)^2+ (y-y_0)^2= a^2, still with radius a but now with center at (x_0, y_0), with the same analysis as before, we have x- x_0= a cos(t), y- y_0= a sin(t) so x= a cos(t)+ x_0, y= a sin(t)+ y_0 are parametric equations.
We can think of \frac{x^2}{a^2}+ \frac{y^2}{b^2}= 1, the relation describing an ellipse with axes, along the x and y axes of lengths a and b, respectively, as \left(\frac{x}{a}\right)^2+ \left(\frac{y}{b}\right)^2= 1 and see that we can take \frac{x}{a}= cos(t), \frac{y}{b}= sin(t) or x= a cos(t), y= b sin(t) as parametric equations for that ellipse.
