I'd like to introduce Mario, one of my students, who is a very talented singer/songwriter. Here are some basic tips that may be helpful for guitarists/singers:
Know you chord changes and lyrics cold!
Practice playing and singing separately first, then when comfortable, put them together.
Simplify your guitar playing - just strum the chords instead of fingerpicking
Feel like you are elongating your neck or "leading with you head" to alleviate shoulder tension.
Try to keep your head up to connect with the audience, also to foster good posture.
Try singing with your eyes closed to internalize the sensations and stay connected in your body.
If you need to sit, sit on the edge of the chair to free up your hips so you can move your torso with the guitar.
Sometimes, making a circle with your nose while singing can alleviate neck/shoulder tension.