There should be several tutorials on Civfanatics on how to do it, that ought to give you the basics. You should also be able to find template files that allow you to copy over Civilizations, units, buildings, and tile improvements to use as a base and modify as you go. If you have any problems I'll be right here to help.

So how do you make your own civ? Because I still want a Gundam Civ but no one has ever made one

I normally use ChatGPT for help

Ah. Never seen Mobile Suit Gundam, so I don't think I'll be able to make a good Civilization for it. I don't like to make Civilizations without being familiar with the source material. The final product would suffer if I did that. Just imagine a Mobile Suit Gundam Civilization made by someone who knows nothing about Mobile Suit Gundam. xD

Mobile Suit Gundam

What's that from? Never heard of it.