Alright, so here's my issue. I'm a vocalist, but I don't play any other instruments. I'm always trying to get a band going, but what always makes it difficult starting out is that even when I do find people who are good and who are serious enough to get a band started, the first time we meet up and have the first little sesh, I'm always expected to jam too. And I have no idea how a vocalist is supposed to jam before knowing how the song goes and what the lyrics are going to be. Anyone know any good ways to practice and jam with other musicians before any songs are actually written, that also would give them a chance to see how you actually sound once you DO know the song?

Learn some standard phrases or choruses or whatever that you can just throw in there when they're jamming.

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or pick a couple songs beforehand and learn the lyrics.

Try some covers first even if only one person knows the song so you can all adapt to each others sound. After that you can start getting some original music and lyrics going. They don't always come at the same time so don't worry if they don't.

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I've always just blurted out whatever sounded like it went with the music, at that point what you're actually saying isn't as important as showing off what you can do. I've gotten quite good at improvising along to bands after doing it for a great number of years and I am called often as a fill-in vocalist for a number of bands local to my area regardless of whether or not I know the songs haha.

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hahaha same, whenever i try out or just jam with a band i completely wing it.

This. I do this when first meeting a band. You must do this :P. I also do this when trying to figure out how the vocals for a song are going to sound-'ya know, to plan out what screams you'll be doing, how you'll do them etc.