To Make your own maps click on Galaxy Forge and.... your off!
To make your own "mods" ie changing values adding new ships I would say Check the links all over the forum stickies.
Basic steps;
-Make a new folder somewhere "C:\mods" or whatever you want to call it.
-Open your Rebellion folder in steam "Y:\Program Files (x86)\Steam\SteamApps\common\sins of a solar empire rebellion"
-Copy the 'GameInfo folder' into your new folder
-Now copy 'msvcr80.dll' and 'ConvertData_Rebellion.exe' and place them In your new Gameinfo folder.
-Right click inside "Gameinfo" folder! select New text Document, Name it EntityBIN2TXT_Rebellion, Open it!
Copy this;
for %%a in (*.entity) do ConvertData_Rebellion entity %%a %%a txt
Save
-Right click it, go to property's at the top in general change the ending of the file name ".txt" to ".bat" and click ok
-Double click this, a Dos cmd window should appear and scroll down.
(skip this is you know where your mod folder is) Run sins, go to Options > Mods > show mod folder;
"C:\Users\name\Documents\my games\Ironclad Games\Sins of a Solar Empire Rebellion\Mods-Rebellion v1.02"
-Close sins, Open this folder
-Create a new folder, call it "Mymod" {or whatever you like} inside this folder make another folder called "Gameinfo"
"C:\Users\name\Documents\my games\Ironclad Games\Sins of a Solar Empire Rebellion\Mods-Rebellion v1.02 Dev\mymod\GameInfo"
-Copy any file you wish to edit from your Mod folder "C:\mods" to here and Open with text editor to fiddle.
Any changes here will effect you ingame.
(If you wish to add a new unit you need to list it in Entity.Manifest which you will find in your sins main dir, copy this into your mod folder and add your new unit on the end)
-Run sins, go to Options > Mods > Enable your mod > make sure you click apply a load screen will flash up for a second.
If you need to Edit Ship models, sounds, weapon particles do the same with those folders
there really are alot of better guides on how to do this... just.... read the forums!
I find http://code.google.com/p/soaseplugin/wiki/RebellionSyntax really helpful for Correct Syntax
