"Carry out any photographic manipulation or processing with your chosen software until you are happy with how the image looks on your computer screen."
This is a huge topic that deserves a multitude of specialised articles. There are many such articles available on the Internet just waiting for you to find them. Suffice to say that, before resizing, you should carry out all the processing your image needs.
It is worth mentioning colour calibration for your computer monitor. If your monitor has not been calibrated to show colours realistically, this can radically alter the way your images are seen when shown elsewhere. For example, if your monitor is adjusted to warm up colours (i.e. a bias towards yellow and red) then when viewed using a correctly calibrated screen or projector the image will seem "cold" (i.e. a bias towards blue and green). Obviously you want others to see your images as you intended, so it is important to have your screen correctly calibrated.