How to find the starting note in your vocal harmony?

I'm in a alternative/punk band and I play guitar and sing background. And the main thing about background vocals are harmonies. I can write great harmonies but the problem for me is how to find that starting note, and also how to stay on track with the harmony phrase so I don't get off track. This is so difficult for me, I've been practicing it and I have gotten better. But are there any tips to give. and yes I have used my guitar to play the note to find it but I just look stupid. thanks