Did you already read your way throuhg the workshops?(http://www.3dgamestudio.com/download.php)

Other than that, I think that you should use models for the sun and the planet. Just create a new empty level in WED and place them in there. Save the level in a new folder somewhere on your hdd. Then go to file->map properties, click on the new button next to the field titled main script (or something like that) and choose "empty script".
This creates a new, empty script named "Levelname.wdl". If your are using A7, you should rename it to "Levelname.c" and change this in the map properties.
Now open that file with SED and write in the mainfunction which will just load your level at the moment and the needed headers:
Code:

#include <acknex.h> //needed when using GS A7
#include <default.c> //not needed, but implements some usefull debugfunctions

void main()
{
level_load("Levelname.wmb");
wait(3);
}


If you now build and run your level in WED it will compile and run this script.
You will be able to move around in your level after pressing 0 with WASD/Arrows and the mouse. Press F11 for the debugpanel, f5 for changing the resolution, f6 for taking a screenshot, alt+enter to toggle between windowed and fullscreen.
You should be able to move around and find your models somewhere .

Now you should adjust the displayingsettings for the "gamestart". Just add the following two lines in front of the level_load:
Code:

var video_screen = 1; //1 for fullscreen, 2 for windowed
var video_mode = 8; //8 for 1024*768, 7 for 800*600, 6 for 640*480 and so on



Just look them up in the manual for more information. To change it at runtime use video_switch or video_set.

Now you need the actions for your planet and the sun. They are ordinary functions but can be choosen in WED and assigned to your models there.
Just add the following lines to your script:
Code:

ENTITY* Sun_ent;
action Sun_act()
{
Sun_ent = my;
}

action Planet_act()
{
while(1)
{
my.pan += 5*time_step; //turn the model around itself
wait(1) //wait until the next frame
}
}



Now reload your Level into WED, righclick the sun, choose "Behaviors" (or choose Properties and go to the behaviors tab) and assigned Sun_act to it.
Do the same for the planet. Build and run your level. It should be the same as before but with the planet turning around itself.
It should be quite easy for you now to make the planet move around the sun.
You can access the planet within the action through the my-pointer
-> change its position through changing my.x/my.y/my.z and its rotation through changing my.pan/my.tilt/my.roll.
Because Sun_act assigns the my pointer to Sun_ent you also know everything about the sun (Sun_ent.x/Sun_ent.y/...) and can now place the planet depending on that.

I hope this helps you a bit to get started
Slin