I am a very happy shopaholic since most brands have gone online. Meaning I can buy clothes without leaving the house.
The only impediment with online shopping is that you do not get to try the items on, so a proper fit is like a roulette run: it may or may not happen. But I've managed to perfect my shopping style and I am also going to share this:
Take measures. That's pretty much it. Most brands have their own ideas about sizes and stuff, meaning not two "S" sizes are alike. In time they actually got this and started to put measurements out for each item, or size charts. Size charts are not as reliable, but I guess they will do if actual lengths or widths are missing.
To take your measurements does not mean you actually have to measure yourself. You just have to measure a garment in the same style with what you want to buy. Meaning you can not measure your duffle coat and expect those stats to fit a slim fit shirt. For example: you want to buy a new leather jacket. Take out your favorite leather jacket, the one you know it fits you perfectly, take out a ruler and get ready to play. Do not measure a bespoke or custom tailor item! Use a stock one if you want the measurements to fit another stock one. Always go from seam to seam with the garment laid flat. For shoulders width go from the left seam to the right seam. For sleeve length go from the shoulder seam all the way down to the end of the cuff. Usually these two measurements are sufficient to ensure a proper fit. There are waist and chest measurements, as well as length measurements if you want to ensure an exact fit. Unless you are underdeveloped or overdeveloped in those areas, you do not really need them. Anyways, the chest is measured from under the arms of the garment, where the lower sleeve meets with the body. Go from the right seam across the chest to the left seam and there's your number. For length you have to go from the top seam at the back, not the top of the collar, all the way to the end of the jacket. Do this across the back of the garment when laid flat.
For any other piece of clothing the measurements are pretty much the same. Do not measure yourself, you will get wrong numbers and then, wrong items.