I don't understand what you mean. An MP3 file has lots of frames and the byte offsets where a new frame begins is always last frame + 1152 (as one frame has 1152 bytes). You should be able to locate the first frame by detecting a sync. word, which always begins with FFF. The first frame is usually located after the ID3v2 tag, so you can skip this tag if present.
I am not sure if the frame size of MPEG 2(.5) Layer III files is also 1152 bytes, but for the most MP3 files (MPEG 1 Layer III) it is 1152 bytes.