selection of proper samples so that peak amp should not change

[delhi]

i have a signal of full sinewave which is starting with a dc and ends with a dc, means it starts with a horizontal line then a full sinewave then again a horizontal line. i m taking its FFT with hamming. now tell me how should i slect the no. of samples and the starting of samples such that if i will shift the sinewave in the hamming window peak value should not change. dc doesnt contribute any change in FFT.should we place the positive peak at the centre of the hamming window, is it necessary?please solve my query as soon as possible.
waiting for your reply.

[DFGray]

If you are starting and ending with a horizontal line, you don't need a window at all, provided the levels are the same on both ends.  The main purpose of a window is to make the ends the same so the waveform is "repetitive."  Yours already is.  Try it without the window.