Translation: Desde la primera vez que te he visto...
Sierra is a Spanish word meaning 'saw'. Since a mountain range resembles a saw with the mountains forming the teeth, it also figuratively means 'mountain range'.
Me viste en la escuela - You saw me in school.
Te vi hoy.
Yo ya lo vi.
vi a su nena
Translation: Lo vi OR Lo he visto.
Translation: Acabo de ver
Since Cheez-Its are a brand name, you just say Cheez-its in Spanish.
Translation: Cuando yo caminaba, vi...
Since your question only concerns the first and third person "was", the Spanish equivalent is "Yo/él/ ella estaba en el teléfono".
Since guacamole is a Spanish food, the word is already in Spanish.
you say it like this: tengo español de primeras