At the beginning of work with squeak I was searching the same things like you. There is no unique way for starting an application (instance of a class). Look in the system browser, select a class an push the button "class". There you can find in most cases the method "new" or in case of gui open. These methods are the entrypoints for you. 
If you want start the application graphicly look to the classes finding in flaps or objects. In the class methods you find the mehtods you have to implement in you own classes starting from flaps or objects.
When you want starting an application at opening the image, look at this mailing backwards. There was an short discussion about that. 
I hope this helps.

Cheers,
   Frank

-------- Original Message --------
Subject: Re: how can I make a thing that can be run? (17-Feb-2006 3:24)
From:    Kendall Shaw <queshaw at pacbell.net>
To:      frank.urbach at schmees.com

> 
> For example, with C there is a "main" routine that is the entry point, 
> so that when you have a C program you "run" it, in one uniform way for 
> all programs (more or less) that causes it to "execute".
> 
> Is there something like that, in these other packaging schemes? Some 
> standard instantiation interface that gets invoked to run your app? Or 
> does the user have to know for each class to open a workspace and type in:
> 
> Blah something: blah somethingElse: blah.
> 
> The Project seems to do something like that, because you could have some 
> GUI thing open that the user will see. Is there another similar mechanism?
>