Voici : from Vois (Voir verb = to see) + ci (from Ceci = pronom démonstratif)
Used to catch people attention on an object.
example : "Voici ma soeur" (Here's my sister *pointing finger*)
C'est : from Ce (Pronom démonstratif) + est (être verb)
It is casual sentence structure (Pronom + verb)
Used to describe a thing
example : "C'est ma soeur" (this person is my sister)
or
"C'est beau" (this is beautiful)
Hope it helps