the labels must be unique. Do not use any LATEX special characters or blanks.
it must be followed by a comma. Also, labels are case-sensitive, so tex is not the
same as TeX.
With two authors, you can put both of their names; with more than 2,
you can add et al. (Linguistically, the use of the Latin et al. in formal writing
follows this rule.) In the first form, if there are two publications by the same
authors in the same year, some people add a, b, . . . after the year (no blank).
In the second form, if there are two publications by the same authors in the
same year, some people add another keyword. You must discover what style
works best for you.
The first declares the bibliography to be in the bib file, mybiblio.bib.
The second command defines the format style of the bibliography to be plain, which comes with every installation of latex. There are other bibliography format styles, including some provided by publishers.