There's really not a ton you need to know going into riven story wise. I'll keep it vague if you care about spoilers but it takes place right after the first game is done and it's basically a brand new mission a character sends you on, and he explains it at the beginning. Puzzle/ gameplay wise however I would definitely recommend playing Myst first. Riven is quite a bit more difficult/ obtuse than Myst, and can be frustrating if you don't know what you're getting into. Myst is a better starting point imo