DON´T WORRY, we can advise you in TWO different ways!
1st way: Have a look at Leonardo´s "Vitruvian
man" and follow these instructions:
a) If you want to buy a jacket, vest, pullover or similar,
measure the perimeter of your chest (C),
waist (W) and hips (H).
b) If you want to buy some pants, measure the length of your
legs (L) from the waist (W).
2nd way: From your own clothes, take the closest
garment to the one you want. Make sure it fits you perfectly.
Then, measure (C, W, H, L) not forgetting to
say that they are garment´s measurements -not your
own-.