Hello Josefa,
Unfortunately, many people skip the gauge step because they want to get started right away. But gauge is really important. A little effort beforehand is better than being annoyed afterward because it doesn't fit.
As a rule, you buy a pattern. In the pattern, the author should have provided precise information about the gauge: which yarn, which needle or hook size, and which pattern were used to arrive at this information.
So you knit or crochet a
15 x 15 cm swatch using the specified yarn, the specified needle or hook size, and the specified stitch pattern. Then you lay it flat and measure—e.g., with the help of a gauge ruler—
how many stitches there are over 10 cm in width and how many rows over 10 cm in height. If your own numbers match the author's information, you can get started. Otherwise, you need to choose a smaller or larger needle or hook size and make another gauge swatch.
If the finished size of a project is crucial, meaning it needs to fit exactly to size, the gauge swatch must be washed and dried. Some yarns distort or stretch out after washing. Then, for example, the garment would not fit.
Best wishes, and have a lovely day, everyone — Ina