Start with a garment that fits you well. Lay it on a flat surface and look at its shape before measuring. Use the same method when comparing it with the measurements in a product description.
For tops, consider the shoulders, chest, sleeves and body length. Think about what you plan to wear underneath: a jacket over a shirt may call for a different amount of room from a top worn on its own.
For trousers, compare the waist, length and leg shape. Consider the fit while sitting or walking as well as standing. The shoes you usually wear also affect where the hem sits around the ankle.
S, M, L and numbered labels are useful starting points. Sizes can vary between garments, and pre-owned clothing may differ from its original fit. Focus on the details of the individual item rather than the size name alone.
If a measurement or detail you need is missing, ask the shop before choosing. After receiving a garment, read its care label for the appropriate washing, drying and storage instructions.