The phase shift is where the function starts. As in, with sin(x), sin(0)=0; it `starts' halfway along an up-stroke.

Basically (assuming you haven't touched the period), if your function starts half-way along an up-stroke, your phase shift will be zero. If its half-way along a down stroke, it will be . If it is at the top of an up-stroke, it will be , which is essentially cos(x), and if it is at the bottom of a down-stroke it will be .

Basically, draw a sine curve, and take a ruler. Put the ruler on the vertical (y-)axis and move it right until what is to the right of your ruler looks like the curve you want. the x-value at this point is what you want.

This will be scaled when you add the period; if the phase-shift (worked out as-above) is c, and the period is d, then the equation you want is .