To choose the destination for your post, do the following:
1. Open the composer
You can access the composer via the green "Create Post" button in the top right corner of the app.
In the navigation bar at the top of the screen, you will see "Posting to":
2. Click the blue text right of "Posting to"
This will open the profile/list selector.
3. Choose a profile or list as your post destination.
TIP: you can use the lock icon to lock a certain profile, set of profiles or list to be always pre-selected when you open the composer.
Just click the lock and turn it green to lock your default profile(s) or list.