One thing that I do for songs that are hard for me to sing and play at the same time... I concentrate on which syllabuls are on beat... then when I pactice it the first couple times I emphasize the syllabuls that are on beat... This helps me keep time and remember the timing of the words vs. the music.... The hardest part for me though was when I started I would concentrate too much on one or the other... I would either play guitar and not sing... or sing and not play guitar... Once I started doing both at the same time... writing songs and such it just all came to me. I sat down and got a chord progression/riff going... and just played it until I could clear my mind and play it effortlessly... then I started humming a melody or harmony along with it... Sometimes I would record this then write words to it... Or, I would keep humming the melody or harmony until words came to me... then I would slowly add in words... Kind of like a freestyle... If you can think of words, sing them, and pay guitar at the same time... you should be alright. This helped me tremendously... I also practiced a lot of easy songs like.... Outside by Staind, Flake by Jack Johnson, Where are you going by Dave Matthews, Angel by Dave Matthews, The Christmas Song by Dave Matthews..... etc... etc... I also learned a lot of Metallica... Fuel, Harvester of Sorrow, Seek and Destroy, Master of Puppets, Nothing Else Matters, Fade to Black, One, Hero of the Day... so on... I learned Hetfields parts to get me use to playing and singing... but I also would learn Hammetts parts...