Interpreting and understanding a knitting pattern can seem like a foreign language. Fortunately it's a universal language that everyone can learn to speak.
Commercial knitting patterns almost always contain a standard set of instructions and information.
The various sizes that a garment will fit are shown, with increasing sizes indicated in parenthesis. Thereafter, the instructions for the particular size you are knitting are given in parenthesis in the same order as the sizes were shown.
The recommended yarn for the project is also indicated. Again, the pattern will tell you how many balls or skeins are needed for the various sizes, following the same order as the sizes. It should also tell you how much one ball or skein weighs and the yardage. This is useful if you wish to use a different yarn.
The gauge needed to ensure the sweater is the proper size is also indicated. Sometimes a row gauge will also be shown, particularly if it is a color pattern and the designer has worked out a pattern that totally repeats from top to bottom. Make sure that you work your gauge over the pattern stitch called for. There's a big difference between stockinette and a lacy stitch.
The estimated needle sizes are given as well. If the needles indicated don't give you the proper gauge, by all means use a different size until your gauges matches that of the pattern.
An easy rule to follow is:
To get fewer stitches to the inch/cm, use a larger needle;
To get more stitches to the inch/cm, use a smaller needle.