I agree with Zé, I want to add try something you know you can sing. When I wanted to start singing I wanted to start with Muse songs which are not easy to sing at all ( Soldier’s Poem and Unintended are easier though) but then I found Arctic Monkeys and my vocal range is pretty similar to Alex Tuner (the signer) therefore I started singing their songs with the easiest chords.
Try to learn the lyrics first, then once you’re comfortable with the chords try both at once. Your voice has to ring with your guitar ( that’s what I’ve been told when I started, that only means sing the notes you’re playing, you’ll feel it when you’ll sing the words).