Brandon's answer is pretty good, but if you're looking for something more advances, you might want to check out linear interpolation: http://en.wikipedia.org/wiki/Linear_interpolation

Also, if your curve is a function, you can know the x and y (and z) coords at a given time. This might help too: http://www.ucl.ac.uk/Mathematics/geomath/level2/fvec/fv8.html#l1

Brandon's answer is pretty good, but if you're looking for something more advanced, you might want to check out linear interpolation: http://en.wikipedia.org/wiki/Linear_interpolation

Also, if your curve is a function, you can know the x and y (and z) coords at a given time. This might help too: http://www.ucl.ac.uk/Mathematics/geomath/level2/fvec/fv8.html#l1