You become a spell caster by well, casting spells. But before you cast any spells I would suggest reading, studying, and gathering resources about exactly what you want to do. There are countless threds here, websites elsewhere, and books that all hold information on what you are trying to do. Most people here are also happy to answer specific questions if you have done some research on your own prior to asking.
Writing spells is another thing, and takes some additional time and effort. Anyways, good luck!