First you have to learn octet rule: every atom needs to have 8 valence atom to become stable. and only the 'inert gasses' have complete 8 valence atoms.
covalent means sharing of atoms. if you look at your table of elements, below every element has either + or - sign.+ means it is missing that many elctron and will attract elements with - to satisfy the octet rule.example carbon has a 4 so he can bondwith 4 hydrogen who only has one.
