There's nothing particularly magic about the commands you use when
writing a package, so you can simply bundle up a set of LaTeX
\(re)newcommand and \(re)newenvironment commands, put them in
a file package.sty and you have a package.
However, any but the most trivial package will require rather more
sophistication. Some details of LaTeX commands for the job are to
be found in 'LaTeX2e for class and package writers'
(clsguide, part of the LaTeX documentation distribution).
Beyond this, a good knowledge of TeX itself is valuable: thus books
such as the TeXbook or
TeX by topic are relevant. With good TeX
knowledge it is possible to use the documented source of LaTeX as
reference material (dedicated authors will acquaint themselves with the
source as a matter of course). A complete set of the documented
source of LaTeX may be prepared by processing the file
source2e.tex in the LaTeX distribution, but individual files
in the distribution may be processed separately with LaTeX, like
any well-constructed .dtx file.
Writing good classes is not easy; it's a good idea to read some
established ones (classes.dtx, for example, is the documented
source of the standard classes other than Letter, and may
itself be formatted with LaTeX). Classes that are not part of the
distribution are commonly based on ones that are, and start by loading
the standard class with \LoadClass - an example of this
technique may be seen in ltxguide.cls
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=writecls