First time DM-ing!!

Hey guys it’s my first time DM-ing and I have no idea how to correctly make my monsters, and what level to make them… I know they need to be appropriate to other players; my players specifically as this is their first session. I wanted something along the lines of a siren, maybe even two…. I might not understand cr completely, but I know one siren let alone two is probably too powerful for around four people starting out at level one… so any advice or help would be much appreciated!! (I’m open to all ideas!!)