Go to ultimate-guitar.com or another guitar tab site and search for the song by title or artist.
Click the song you want to learn and the tab will open up.
Look at the letters on the sides of tabs. These represent the names of the strings. The numbers that are lined up with the string name represent the fret (for example: A | ---3--- | means to put your finger on the third fret of the A string).
Place your fingers on the guitar in accordance with the tablature's instructions. If there's a 3 on the low E string, a 2 on the A string, a 0 on the D, G and B strings, and a 3 on the high E string stacked directly over each other, that is a G chord. So you put your fingers on the third fret of the low E string, the second fret of the A string, and the third fret of the high E string.
Study chords until you get the hang of them, then give a shot at leads. Leads look almost the same as chords, but they won't be directly over each other. Instead, the numbers will be one after the other. The tab can show you the sequence of notes, but it can't teach you the rhythm.