The best thing to do (and you need to do it anyway) is to download the modding package, you can find it here:
WWW LinkRead up on the pdf help file, it has a few starter pointers as to how things are changed and such.
After that, check out files PlayerPhase.entity (Vasari), PlayerPsi.entity (Advent) and PlayerTech.entity (TEC) to see how they are done. Basically, if you want to just change how things are called, you will also need to edit the english.str which is located in your installation directory in the \string folder. You can also open and edit that with Notepad.
Modifying a race can get fairly complex, because there are multiple files involved, depending on the task at hand. For example, if you want to change the name of the Vasari race to "Badass Muthas" you only need to find the IDS_PLAYERRACENAME_PHASE entry in the english.str and change "Vasari" to "Badass Muthas". Voila.
But, if you want to change research topic descriptions, or the names of various ships and modules, you will have to identify files which govern those items and then find ID string designations there, after which you find those in english.str and edit them.
For example, changing the name of the Vasari missile battery would go as follows:
In the PlayerPhase.entity file you can find the following line:
entityDefName "PLANETMODULE_PHASEORBITALMISSILEDEFENSE"
which means you have to open the PLANETMODULE_PHASEORBITALMISSILEDEFENSE.entity file next, where you will find the following two lines:
NameStringID "IDS_PLANETMODULE_PHASEORBITALMISSILEDEFENSE_NAME"
DescriptionStringID "IDS_PLANETMODULE_PHASEORBITALMISSILEDEFENSE_DESCRIPTION"
The first is the name of the Module, the second the description which appears on the build menu infocard. Opening the english.str, find those two values and change their values in order to have something else appear.
This is how you alter descriptions and names. Altering various capabilities (for example making the missile battery fire twice as fast) requires editing of appropriate values in appropriate files. Adding new models and textures is even more complex than that, but for starters I suggest you simply familiarize yourself with the way the system works - its intuitive really, not that hard to figure out.
Good luck!