Nonmetals form which type of bonds by sharing electrons to complete their valence shell and increase stability?
These are called co-valent bonds.
Say two flurine atoms meet. They both have 7 electrons in their outer shell. If they share one of those with the other atom, they each "appear" to have 8, which is a stable situation.
(my chemistry teacher used the metaphore of two polar bears pushing the bald spots in their skins together)