Watch the Basic Vibrato online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
Vibrato is every player's signature expression. It's one of, if not THE technique, that will separates you from everyone else. Vibrato can be applied with your fret hand finger of choice in a few different ways such as a horizontal approach, a vertical approach or a circular approach. Sometimes moving the neck or even grabbing it by the headstock and bending it can provide you the pitch modulation you're looking for.
Looking for inspiration? It's almost universally agreed that B.B. King had the vibrato to end all vibrato. Truth be told, vibrato is a subjective thing, so only you can decide what type or who's vibrato floats your boat. Whomever it may be, the best advice I can pass along is to put the time in emulating their vibrato until you sound just like them!