Making a gauge swatch is the best thing you can do for your project.
Many knitters/crocheters dislike making swatches and think of them as a waste of time.
On the contrary, making a swatch saves you time and facilitates making a better fitting garment.
Usually the pattern or the yarn label gives you the appropriate gauge (tension) to work with a specific yarn. The gauge tells you how many stitches and rows you should have per inch/cm using a certain needle/hook size.
To determine your gauge, knit/crochet a sample swatch a bit larger than 4"x4" (10 cm x 10 cm) with your intended yarn and needle/hook. Block the swatch the same way you block the garment. Place it on a flat surface. Position a ruler or measuring tape horizontally on the swatch, a couple of stitches from the edge, and measure 4" (10 cm). Count the number of stitches including partial stitches. To measure the row gauge, place the ruler vertically, measure 4" (10 cm), and count the number of rows.
Compare your gauge with the one given in the pattern or on the yarn label. If your gauge matches the one specified, you are ready to start. If you have fewer stitches, your gauge is too loose, try using a smaller needle/hook. If you have more stitches, your gauge is too tight, try using a larger needle/hook.