A few on-line introductions to Mathematical Induction:

1. http://en.wikipedia.org/wiki/Mathematical_induction This Wikepedia article is a very good introduction.  Read the first part (up though the section entitled “Polya’s proof”).
2. http://www.themathpage.com/aPreCalc/mathematical-induction.htm Another good introduction, with a few simple examples and some problems to check your understanding.
3. http://www.cut-the-knot.org/induction.shtml  A simple explanation and example with links to many other examples (a few of those examples are quite advanced).
4. http://www.math.csusb.edu/notes/proofs/pfnot/node4.html  A page about proof techniques in general, of which Mathematical Induction is one.
5. http://www2.edc.org/makingmath/mathtools/induction/induction.asp A brief intro with a couple of standard examples and simple explanations