Being part of a coven can help you learn from the experience and knowledge of others, and thereby you may become more skilled yourself. But of course it isn't necessary to be part of a coven to do that. Being part of a coven also permits you to be part of a group of like-minded people which can be a lot of fun.
As to the last part of your question, you can't start a new coven on this site. The site owner feels that there are already enough covens here to meet the needs of the members. You might want to check out the existing covens to see if there is one that appeals to you.