And you had better do this while it is still TIFF or PSD file, as not to work on a jpg (every time you save a jpg it looses information)
Go to Image>Image size
Type 750 on the longest side. If you have checked the three boxes at the bottom, that is all you need to do about size. The proportions will be kept.
Make sure your image is in sRGB color mode for best web display.
If I remember CS, it was under Image>Mode, I may be wrong there because they keep moving this command around.
The go to File>Save for Web and look at the weight (kb) in the bottom left. If it is over 145 kp, go to the quality slider, upper right, and move it until you reach the allowed weight.