The voice is the first instrument we learn to play and express ourself with. It operates under the same fundamentals of music. Pitch, rhythm, melody, harmony, dynamics, etc. It might not sound pretty but we all can sing. Ever notice how all the jazz cats sing the notes during their solos?
Take a sound (start with a riff or lick) from a song, sing it (focus on the rhythm and melody over the pitch of your voice), and play it on the guitar. In time you will develop the skill, your ear, and your voice as a musician.
"If you pay attention – what I sing, the guitar sings too, and what the guitar says, I say." – Fred McDowell
All the legends sing through their instrument. They connect their voice to the guitar and make you feel something.
Mississippi Fred McDowell was an hill country blues singer and guitar player. He was particularly renowned for his mastery of slide guitar, a style he said he first learned using a pocketknife. He later settled on a glass slide for a cleaner sound.