Guitar tab is one of those things that once you know how to read it, you never forget it. There are six horizontal lines. The bottom one is the sixth string, the low E, and the top one is the high E. The rest are just the ones found in-between in order. The numbers on the lines only tell you what fret to play that note on, but not what finger to use. The writers of tab expect you to figure out the best way to play the notes they wrote.
Take at look at this: it is the beginning of 'Happy Birthday'…
To play this on guitar, you would fret the B string at the third fret and play it, play it again, then fret the same string at the fifth fret, and so on. Even if it is not marked, the time flows at even intervals from left to right. The last thing represented in the tab above is a chord, in this case the D7 chord. You have to fret and strike all of the strings marked at the same time. In this case it's pretty easy because two of the strings are 'open' - struck without being fretted.
There are many places online where you can find guitar tab.