@Summum great answer!
@holli Do you know it what way you are terrible? For example, do you manage to hold the tune but get the timing all wrong? Or is your timing fine, but your tune a bit off?
Recording yourself so you hear how well you fit with the piece is a great idea…
If you find that you’re singing flat (ie, not quite reaching the right notes but singing just below them) then try smiling as you sing, and try falling onto the highest notes (imagine you’re about to sing higher than the note and then come down to it, rather than reaching up to it)...
Timing is more a matter of practise and really knowing the song very well…
I would recommend finding a song that doesn’t feel like it strains you to sing it… so it doesn’t go too high or too low for you, and it doesn’t go too fast or have too long notes for you… Then practise it like billy-o!
Good luck!