Everything you need to learn salsa can be found on SalsaNotes.com (see link in the references section of this article). The site has an excellent beginner's tutorial, which you should read through if you've never danced salsa. You will learn the basic steps, the role of the two partners, the rudiments of salsa music, as well as some dance etiquette, all on a single page. Practice as you learn---all you need is some music and a partner.
In salsa dance, a "routine" is a series of dance steps memorized and performed in succession. SalsaNotes has a database of more than thirty routines of varying difficulty levels. Practice each step in a routine by itself several times until you are able to do it properly before trying to go through the entire routine in order.
Once you're comfortable with the basics of salsa and have mastered a number of routines, construct some routines of your own. There is an enormous list of salsa dance steps and moves on the SalsaNotes website. The steps and moves are categorized by the hand positions they start and end in. Each step has a detailed chart explaining how to perform it, making learning new steps very easy. To make your own routines, string together a series of moves with matching starting and endings.