i got the strumming down i dont know when to sing

You're making progress, as I see, that's great.

You can play that intro you're playing over the whole song.

See here:
http://www.chordie.com/chord.pere/www.p … OCKING.TXT

This version is formatted quite nicely, that means that it gives an idea of where to change the chords while you sing the lyrics.
Exactly when to start singing depends on what strumming pattern you're using, and you're probably going to change that pattern with every line of that song very soon, that's JAMMING, YEAH
Unfortunately, the song above is a little off in the first line, so here's where I would change the chords:

G           D                               Am              Am
  Mama, take this badge off of me
G           D              C            C
  I can't use it anymore

I suggest this:
In order to get a feeling about how the singing ties in with the guitar playing, you could sing the song to yourself and just play the very first downstrum of each new Chord. You can get an idea of when to place that first downstrum from the link above, but you'll also recognize that by yourself while your singing, because your voice will make the same changes in pitch. I will just "feel right".
Now, in the two lines above, I indented the lyric lines a little while the Gs on the chord lines are the very first character of the line. That means I would Downstrum the G, the start singing the "Mama...". The other Chord symbols are right on that syllable where your first downstrum of that chord should be. Note there's two Am and two C, because you play those chord for two full measures.

Once you're comfortable with all those first downstrums, you may want to bring in the other down- and upstrums for each chord while you keep singing. You sort of tie all those first downstrums together with whatever strumming pattern you want to use.

Basically, singing and playing is something you will get comfortable with. I remember watching someone freely strum along to loads of songs at a party. I had been playing for just a few months by then and I was still to get comfortable with very basic chords.
So I tried that at home and it seemed impossible. I thought I would never be able to sing and play at the same time, but I kept trying with simple songs. I had a whole load of songs printed out by then, and it literally happened over night that I could play that one song I was failing at the whole time. To my surprise, I could strum along to pretty much my whole collection of songs all of a sudden.

That's just how it happens, keep trying, you'll get it.
A good thing is to watch others play, so remember searching for "acoustic cover", "acoustic lesson" or sth similar on youtube, along with whatever songtitle you want to see people play.
Helped me a lot.

Hope it helped a little and I didn't confuse you too much

Cheers,
nc