If you arent born with it , you will be sooooo much better off with lessons.
But here you go anyway
Sing Julie Andrews' "Doe-A-Deer". From the musical the Sound of Music.
It is a song made up of the 8 notes of an octave but the first and last notes are the same note, only one octave apart.
Ill start it off.... wait..you play guitar..you should know this one by now.
Do a deer , a female deer.....Ra , a drop of golden sun....Me, a name , i call myself....Fa , a long , long way to run...So , a needle pulling thread.....an so on..
This you can sing in any key. You can start on any note...just go thru the scale..do ,ra, me , fa, so , la , ti , do...
This isnt going to be what you play for your friends but its a great warm-up and if you do the scale with your guitar and follow along with singing then you will know if you are off key.
hope this helps.