I think an important thing to do is to realize what you want to write, and what you are qualified to write. If you can find a marriage of those two things, then you have put two important pieces in place. The best advice I've ever received is write constantly, and I'd only append it to add take into account your desire and ability.
To get into it as a career, realize you must be patient, that it may take ten years, and take that time to grow. The most successful writers are keen observers of human behavior as well as masters of language. Studying life, as well as your craft, can only help you fine tune your native abilities.
As to whether or not you need to do one specific thing or not (like join a writing group, join a writing site, find an agent), keep in mind there is more than one way to skin a cat. What works for you, works for you. If someone tells you it doesn't work for them, listen to why it doesn't work and use your own judgment.
Most of all, learned to be satisfied with the act of producing your writing because if you are in this for recognition or money, you'll forever find yourself needing more. The only way I've learned to have true satisfaction as a writer is when I am proud of what I have written for reasons I understand. This is what gives me belief in it in the face of rejection and opposition. This sustains me. If this sounds like something you can identify with, then you will have the tenacity, discipline, and patience for the readers to come.
Hopefully some of this helps! :)