Local branches within the SCA come in three basic types. A shire is a local branch that is part of the Kingdom (and of the applicable Region), but not part of a Barony. A canton is a local branch that is part of a Barony as well as the Kingdom and the Region. A college is like a shire or canton, except that it is allowed to "go dormant" during the summer school recess months.

There are other types of local group, but these are the most common. In some areas you will hear some groups referred to as the "Marche of Something-or-other". The term "Marche" has no official meaning in the SCA organization, and is simply an additional part of the name based in some traditions that were established years ago.

A barony is a larger group that has "landed nobility" in the form of a Baron and a Baroness who "hold the land in fief to the Crown". Baronies can be the most-local level of the organization in some cases -- typically a large city -- but also can be a "shell barony" that contains cantons within it. It is highly unlikely that you would be allowed to form a barony out of thin air in most kingdoms; generally, baronies are formed when existing local groups grow and thrive over a period of years.

A region, which does not exist in all kingdoms, is purely an administrative division, because many Kingdoms are too large to manage as a single unit. A region has officers who are technically deputies of their corresponding Kingdom officers, but a region does not have any landed nobility or other in-period officials.

If you are interested in forming a new group, then you will probably be wanting to form a shire, a canton, or a college, depending on your local circumstances.