To sing and play guitar at the same time takes some practice. First, you need to memorize the guitar part of the song. Before trying to sing while playing, you need to be able to play the guitar part without thinking about it. Then you need to be able to sing the song without having to think about it either. Finally, after being able to play the guitar without thinking about it, and sing without thinking about it, try to put it together. The reason for all of the memorization is that if you have to think about fingering a certain chord, you'll end up missing the lyric, or if you have to think about the lyrics, you'll get distracted from the guitar. And just remember that practice makes perfect.

Hopefully you understood all of that. Otherwise, you may be able to find some advice in the vocal threads, found here: https://www.ultimate-guitar.com/forum/forumdisplay.php?f=83 Best of luck to you
From experience... I would try humming whilst playing first and watch what you play on guitar! Eventually it will become easier and you will be able to belt it out whilst playing!

You have to sub-consciously know what you are playing on guitar first, then you will be able to improvise whilst singing like Jimi Hendrix
When the power of love overcomes the love of power the world will know peace! - Jimi Hendrix