Most guitar music available on the internet these days is written out in guitar tab rather than music notation. Here is a brief explanation on how to read it with a downloadable PDF to keep as a reference.
In a brief overview, reading tab is very simple. There are 6 lines that represent the 6 strings. The top line is the high E (thinnest string) and the bottom line is the bottom E (thickest string). The numbers written on those strings indicate what fret you should be pushing down on that string. For example, the number 3 written on the bottom line means you need to push down Fret 3 on the bottom E (thickest) string. A '0' written on a line indicates that you need to play that string open (not fretted).
With that you can download most tabs and start working them out easily. There are lots of extra symbols which are explained in the picture below and in the downloadable PDF. I have also put a link to a warm up for you to download to work out and practice reading tab.