King-

You can start by singing scales along with your guitar. Start with the lowest note you can sing comfortable and just work up from there. Then once your get familiar with the scale, try singing the note before you play it on the guitar and check to see how accurate you were. This is will help build accuracy with your vocals. Definitely do what simon said about recording yourself, that will help you recognize and correct faults in your singing. Takes some time if your not used to sing, so be patient. best of luck.