Me and my band just started and I want to come up with some cool vocal melodies. Nothing too AMAZING. But whenever i come up with one in my head I can never remember it. Anyone have tips on writing vocal melodies? Could I write it on guitar and then translate that to the lyrics?

Music notation is generally used :P It can take a while to learn though and to be able to translate it from your voice. I only really just started learning it about a couple of months ago. I think the best way to save something once you've come up with it is just record your voice or the band's singer with your phone, etc
Totally get in the habit of recording rehearsals. Best advice ever.

Fleeting magical moments do happen, and often, if you don't record them, that's it. Gone. We've learned that the hard way a number of times now.