The best first songs to sing are nursery rhymes.
Other than that, see if you can do the standard scale do-re-mi-fa-so-la-ti-do and record it and see how that goes.
Then after hearing yourself do the do-re-mi... I would recommend being able to sing along to this. Do it where you are able to record yourself and compare, so you can mess around with your voice. To be honest, you just have to sound as good as the kids..
