You may start a coven when there isn’t anything else local and yet there are many local Pagans. Or, you might start a new coven when it branches away from an established coven.
Starting a coven of your own is not something you should take on by yourself. Don’t try starting a coven of one, that’s not a coven, it’s a solitary. Also, you will need help, other view points, sources of knowledge and experience too.
Narrow down a focus for your group. This is very important if you are in an area where there are already Pagan groups (covens, circles or casual groups). If there isn’t an existing group for Pagans you still need to start with a focus, a statement of purpose. It’s not so easy to be completely open, in spite of good intentions it doesn’t work so well in practice.
Decide how the coven will be run and managed. How many members will the group be limited to? How will requests to join be handled in the future? Should everyone be initiated and what will the initiation be? Will there be rank (including a High Priestess) or will it be casual? Will the group meet once a week or once a month? Will the group have extra gatherings to honour the Pagan holidays? Will the group raise money for causes? There are endless small and large things to decide and talk about.
Choose a leader. Even if the group is casual someone will need to start rituals, open and close circles and so on.
Choose a place and time to meet. Consider available transportation: bus, parking for cars? If you meet in a public place get permission from the owner/ manager, don’t assume it will be ok. If you meet at someone’s home who brings the goodies, if there are goodies being brought?