If you don't have any saved sectors, press the + button in the toolbar to create a sector.
Well, that's boring. On that view, press the Edit button in the toolbar to make things more interesting.
If you want all new worlds to start with a specific allegiance, press the blue default allegiance button at the bottom left of the view. In that view, you can create a custom allegiance of your own or use one of the standard Traveller ones already defined for you.
If you want the app to randomly generate worlds for you, then you've got some options related to Stellar Density, which is the odds of a sector hex containing a world according to the standard Traveller random world generation rules. SectorMaker lets you have a different density for each subsector. You can let the app randomize this for you, set the same density for all subsectors with the slider at the top, or set each subsector individually with its own slider.
Once you've done that, press the Generate Random Worlds button, which changes to a Delete Worlds caption. Return to the Sector view to see the results. Once you've got at least one world created, the Stellar Density controls no longer apply and are locked out until the last world has been deleted.