It is very important to check your tension before beginning to knit, to ensure your finished garment fits you as you want it to.
Your tension should coincide with the tension given in the pattern, otherwise your garment may come out one or several sizes smaller or larger.
To check your tension, cast on 40 sts on size 4mm straight needles and knit 44 rows in stocking stitch. Then check with a tape measure that you have the right number of stitches and rows as indicated in the pattern.
If your tension is too loose (too few stitches per 10 cm) use needles that are half to one size smaller and if your tension is too tight (too many stitches per 10 cm) use needles that are half to one size larger.