Place the cursor at a point near the origin and click the mouse button to mark an initial vector. The x and y components of that vector will be the vx and vy values in the parametric equations,
x = vx * t
y = vy * t - 4.9 * t^2

As you can see, x and y are both functions of t so for any value of t we get an (x,y) ordered pair which defines a position in our two dimensional space. We call our initial time t=0 and pick a t of .01 seconds. When you mark the initial value of vector by clicking the mouse button, you set the values of the the vx and vy parameters. Then the value of x and y is calculated and time is advanced by t repeatedly until the "trajectory", the path of the particle, returns to y=0.