I am just learning to play guitar and I want to master singing at the same time as playing. I have a really hard time keeping two separate beats though. I will start to sing and then I loose it on guitar, my guitar strumming will match how I am singing. So yeah, if anyone can help me, please do 
I honestly had the same problem when I started!!!! I found you get better with time. I think if you play along while listening to the song it helps – just play along and concentrate on listening to the lyrics and how they fit with the strumming pattern of the song. Do this for a while and then try and play the song on guitar while keeping rhythym strumming and try and do the lyrics in your head while playing. Apart from that just practice. You can also try and slow down your rhythym or only strum acouple of times or once and then sing while letting your guitar ring out. Please visit my site www.getguitar.hpage.com
Theres lots of easy songs to learn 1x added daily and a great way to learn is watching the videos. you can play along while watching and follow the rhythym and lyrics too!