When I sing I sound very monotone. Is there a way I can learn by myself to get a decent voice without damaging my voice? Nothing spectacular, but enough to sound good with my guitar playing.
I would strongly advise you take lessons because I found I couldn't really learn by myself. But I do have a couple of tips: Try to stand or sit up very straight while you're singing, breathe from your diaphragm (that's around your stomach/intestinal area), and most importantly, think like you're singing out of the top of your head. You cannot get a decent voice while damaging it. If you damage your voice and don't sound horrible you were probably singing correctly before and then forgetting to do certain things.
A reader's digest version of good singing...

1.) Breathe so your belly expands.
2.) Don't let your voice get caught in your mouth; let it rise into your head.
3.) If your voice cracks, SING THROUGH THE CRACKS. That's the only way your voice will get the muscle memory necessary to regulate levels of airflow.
4.) Don't push.
5.) Sing in the front of your face.
It probably sounds monotone because you're more speaking on pitch than singing. Do some vocal exercises where you try to hit particular notes.
And no, Guitar Hero will not help. Even on expert. Really.