Most guitar tabs I've seen contains a chord notations(in the top of the tab), but I don't know how to use them. For example I'm reading Love Story Francis Lai, Should I play(Em,C,B7 and Am) chords also when I'm reading the tab?
What you're playing in the tab directly relates to the chord above it so if you are playing the tab right you are already playing the chords in some manner. If you're starting to learn theory this should help you understand the basic chord progression of the song.
It's really letting you know what all the notes you're playing equate to as a chord. I know it is easier for me to think of a chord or a chord shape rather than play a bunch of notes that don't equate to anything. If you look at the first bar for the tab you got it pretty much looks like an E minor chord with a few extra notes, which is what the
Em is telling you.
In short just play what is on the tab and you should be fine.
A disclaimer about guitar tabs: I would not trust any tab 100% since anyone can post one. I myself have come across many tabs that are rather inaccurate, but I have also come across tabs that are perfect. I don't want to discourage you from learning just want to give you a warning.