What Hugh said...
Actually, if you don't know later Trek that well, I'd say it's probably one of the best eras to start in. Nothing's really fixed yet, and you have lots of room to manoeuvre in without treading on canon.
The one thing you have to be careful with is technology. Enterprise is definitely low-tech compared even to TOS, so be careful to keep that feel. Slow ships, no real transporters, limited sensors, clunky spacesuits, aliens that are much more powerful than humans...
Would be fun!
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)