Singing fancy bluegrass or pop harmony is difficult. Singing songs like operas or The Star Spangled Banner is hard. But singing Clementine or Tom Dooley isn't that difficult if you match the song to your voice.
Since you're just learning what the guitar or mandolin or dobro or banjo or whatever sounds like, you're in a good spot for changing it to match your voice before you get too hung up on the "proper" (i.e. familiar) sounds of songs.
And try to choose a song with long notes such as Amazing Grace or Swing Low Sweet Chariot or Tom Dooley. If not, then just select a children's song such as Polly Wolly Doodle or Skip to My Lou. Or Yankee Doodle. Battle Hymn of the Republic.
Anyway, sing the song when you're alone in the car or shower or where ever.
DO NOT PLAY IT ON ANY INSTRUMENT—it will mess up your ear.
Once you're completely happy with how it sounds (which may take several weeks), then get out your stringed instrument (or tuner) and sing the last note and figure out what that note is. That's the key you ought to:
After a lot of good sounding singing on different songs, your voice will be stronger and you will be more confident with it and you can then move up or down one or two steps and still sound good when you play in a jam in "regular" keys.
A rule of thumb for acoustic group audiences used to be 25 listeners/musician. A string quartet could play for 100 people. I can play unamplified for about a 100 attentive listeners (not bar patrons) in a gymnasium, and about 50 in a carpeted room or outdoors.