Learn some nirvana tunes, those are a good start.
Exercise steps.
1. Play the rhtyhm to the rhythm of what your singing. So your strumming your guitar to every word or syllable. You don't even need to know a song, just say stuff at random like "my ex girlfriend liked to have sex with dogs" if you want. That should be easy to get used to. Now try it with a metronome, sing and strum on every click.
2. Keep try singing on each click while cutting up your rhythm, say instead of strumming on each click, strum twice for each click. Do the opposite, strum on each click and sing 2 syllables for each click.
3. Once your get that done, mix it up. Strum a rhythm and sing a different rhythm over that one. If you need to strum a rhythm for a few measure first, then come in singing a rhythm thats cool.
4. Once this becomes alittle easy, try learning songs. Go for pop, punk songs. I wouldn't start with something like blues because that can be alittle too advanced.
Since most singer/guitarist, sing first, and strum an easy rhythm on the guitar to keep time. Once your able to keep a tempo with the guitar, singing will come pretty easy. You may just be trying to do too much. You can get to the point of singer/lead guitarist but that takes lots of practice. Just start simple.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.