I would take a look throught he starting tutorials on the wiki first and foremost:
https://planetside.co.uk/wiki/index.php?title=Terragen_TutorialsFor example here is a Realistic Earth Tutorial by
@sboerner https://dotsandlines.steveboerner.com/2016/07/08/terragen-tutorial-rendering-a-realistic-planet-earthI also suggest Modeling in Terragen series by Vladimir Chopine
https://www.youtube.com/playlist?list=PLaH1PMiI_7AhJ44dm9tnGa-ZQpXNOY6ivHere is a 10 year old tutorial from youtube
https://www.youtube.com/watch?v=E4ZaYStKTtY Though I suggest using a Sphere for a global ocean. You can do this in Create Object -> Sphere. From there, go into your planet node, copy the planet Center, and paste it into the spheres Center. Than copy the planet radius, and paste that as the sphere radius. You now have a sphere at the planet radius, so, you can call the default displacement of 0 sea level, and any negative displacement would fall under the ocean. In the sphere object uncheck "Cast Shadows" and as the surface shader you can go ahead and Create Shader -> Surface Shader -> Water. From there you can open the water shader and customize it. I suggest a larger decay distance under Sub-Surface tab.
As for stars, we just really use the sun for that, and post processing, though I have attempted both a solar system and star of sub-par quality (though I was using freeware and there was a limit of fidelity I could achieve to even see what I was doing)
Also, definitely browse the file sharing sections after you've looked through some tutorials and interacted a bit with TG to familiarize yourself again.