I usually say the first thing you gotta learn about music theory are the rudements. Time signatures, tempo, notes, and then you move to intervals then chord structure.

Anyway, it's more like this

I ii iii IV V vi viidim

The captial ones are major the smaller one are minor. Never write III unless you really mean the 3rd chord is major. A lot people will think you went to the relative minor of your major and you start confusing people. So it's good practice to remember captals are major lower-case minor. the 7th in a major is dimisned and the 2nd in a minor is dimished.