Please understand that I haven't found out how to use subscripts using this program
so water is referred to as H
Covalent bonds happen between two non-metals.  One of the most common of
these is water (H
2O).  First lets look at the two atoms involved.
Remember, hydrogen fits well in either group 1 or group 17, in the case
of bonding it behaves like a halogen (group 17).  This means that can be
considered to have space for one more electron in it's outer shell in order
to have a full set or it can lose one electron to have an empty shell!

Notice that the
nucleus is not
shown, that is
because the
doesn't effect
the bonding.

With 6
electrons in its
outer shell, it
has two
spaces to fill.  
One on the top
and one on the
bottom in this
Covalent Bonds - The Atoms