When creating or editing a team, there will be a "member of" drop-down available which will list:
* The season that they were created in
* Any (house) Leagues created in that season
* Any (house) League Divisions created for any leagues
(note - the divisions for a house league is rarely used, it's only for house league age groups with a large amount (10+) of teams)
So to assign a team to a league, simply change the selection in the "member of" drop-down. This is done by going to System Administration -> Manage Seasons -> Setup Season Members -> (select the season), and then double-clicking on the team.
Note that the leagues have to be created first in order to assign a team to that league.
The other option available when adding / editing a team is the "category" drop-down which is typically used to group collections of non-league (ie travel) teams.
Hope that helps,