Well, do you know everything that you'd like to change at least? If you are only wanting the new race to have different ships for example it is quite a bit easier than making a custom tech tree and such. No matter what though you'll need to copy and change a Player(race name).entity file in the gameinfo folder as well as add default templates for you race in the GalaxySenarioDef file. You also probably need to make some additions to the string file but you'll probably want to wait on that until your new race is a bit more complete.

In this thread I summarized the steps I went through to add a new race into sins barebones with a single custom ship. This is a very generic, quick breakdown of the steps, but it should give you an idea of what's involved.

https://forums.sinsofasolarempire.com/374437

As I discovered in that thread, you'll need updated files converted from sins itself to avoid major issues instead of the ref files contained with the tools.

No Problem

"EDIT" Btw i'm trying to promote my mod and get some more viewers so ill just leave this here

"EDIT" "EDIT" Stupid Moddb embed code sucks ill just leave a link -- http://www.moddb.com/mods/sins-of-a-solar-empire-revolutions