Season 1 Episode 1 and 2- Welcome to the Hellmouth and The Harvest
We open Welcome to the Hellmouth with two teenagers breaking into the high school, only to find out that one is a vampire. From there we see Buffy having nightmares and discover she's psychically linked to the Master vampire.
Buffy has a moment when she talks to Giles about loosing her friends, being kicked out of school for what they considered bad behavouir and suddenly Giles becomes aware that life as it's been known can't ever be replaced.
Cordelia has the best line in this first episode "Gym was cancelled due to the extreme dead guy in the locker" It sets the tone for the whole series, drawing the dark humour out of the depression.
In The Harvest, we start to see the baseline of what the talents are each member of the group brings. Willow has the knowledge, Giles the quiet authority, Xander the strength to fall back on. While Cordelia and Angel at this point are sort of just along for the ride. Both representing bookends of the situation, Cordelia is "reality as we know it" and Angel is "the real reality". You could almost say they also represent the gang's past and future.
This was a two part opening episode.
What do I think the main themes in these two episodes are... transitions from who you thought you were to who you are meant to be. The setting of high school is the metaphor itself more then anything else right off the bat. Buffy knows she's has no choice but to face what is in store for her, but isn't excited about it at all.
Cordelia is the familiar safe iconic side of Buffy (past) and Angel is the unknown scary side (future)