In the spirit of "teach a man to fish": Your song appears to be in the key of C based on these chords. In the key of C, C is the "I" chord, F is the "IV" chord and G is the "V" chord. (D is "ii" - lower case for minor - E = "iii", A = "vi" and B is "vii"). The numbering system works in any key (if your song was in the key of D, D would be "I", G would be "IV" and A would be "V"). It's not based on which chord you play first, is based on the harmonic relationships - C F and G are the core scale tones for the key of C. I know this because of lots of theory training.
This is called the Nashville number system and is used by a lot of musicians to communicate chord relationships. It is particularly useful for situations where a song may need to be transposed on the fly. If you under stand your "I IV V"s, it's real easy to transpose stuff quickly. You'll also find this referenced a lot in theory discussions, because in most cases, the actual key is not important, it's the relationship of the scale tones to one another.