I got a guitar a few weeks ago and have gotten familiar with it. It's tuned and everything, but I would like to play a cover for a song with my acoustic guitar. However, when I look at tabs for a song I want to cover, I get really confused (link of the song with tabs below).
What is tuning: Capo 3? And what is Em7 and Cadd9? If someone could please explain this to me that would be great!

you need to learnt he basics
and capo 3 is that you put a capo on the third fret(you need to buy a capo)
and em and cadd9 are chords if you put your mouse ove the blus chord it shows you how you can play them
You need device called a capo, which "floats" the start of the fretboard around, allowing for vocalists to sing a song in a key that's comfortable for them. You can pick one up for about $15.
I can only listen to so many breakdowns and "spoken word" vocals before I wanna puke.

I find Jennette McCurdy attractive, but Elizabeth Gillies and Debby Ryan much more so.

That's enough, Djent people. We get it.
capo 3 means placing a capo on the 3rd fret.
Em7 and Cadd9 are chords. you can google both capo info and chord's.
in fact, downloading a free guitar pro/powertab reader such as tux guitar will show those chords in a tablature form, which is much easier for novice players.