As a member of a Space, you interact with it by using the widgets that the Space administrator has configured, and by using the buttons provided by the widget in the Action Button Bar.
See: Using Spaces home screen
The left sidebar shows the widgets. In the Welcome Aboard Space, these are presented in no particular order, though if a widget has a name like Welcome, then logically that is the one you should look at first:
If the administrator has designed the Space to lead you through a process, they may have grouped the widgets into sections or phases. You should go through the sections in order, completing the widgets in each one before proceeding to the next.
See: Widgets example
You interact with a widget by clicking on it in the left sidebar to expose its content in the work area. Many widgets are quite passive, and simply display information for you to read or view, but others may send you to an interactive web page, or ask you for feedback, or allow you to contribute to a common document, along with the other members of the Space.
See: Widgets reference
Note: The administrator gives the widget a name when they create it, so the icon may be the best guide as to what sort of widget it is, and how to interact with it. However, the administrator may also change the icon, so you cannot rely on this completely.