I just finished a jam session and maybe this will help you.
1st, we all show up with songs we know how to play, so having "go to" material is something everyone does and needs.
We all have licks as well. If you can make a solo up on the fly that is great, but I would start by having some "go to" licks. So I would have some of these down. An easy one to start with is the solo from " You Shook Me All Night Long". Take the 1st half of the solo, when you are jamming to G Minor, any G Minor, start playing in time to the groove. Your friends will think your a genius and have no idea your paying AC DC. You can move this around as well, jamming to A minor, move the whole thing down 2 frets. Again, your friends will not know your playing AC DC or whoever.
Want to make up solos on the fly. It's not hard. Ask what key you are in, start playing a pentatonic scale. Listen. Do you like the notes? If you do keep playing them, if not, don't play them again. You will hear about "augmenting chord changes", that just means if the chord switches to an "A" you are hitting the A note at the same time. Just practice your timing and it will make sense. Your ears will tell if you got it right.
If you want to make up a song on fly, easy. Go to "chords in the key of" on Google. Pick the 1st, 4th and 5th chords in that key. Start strumming. Make stuff up. Do you like it? Do you like the strum pattern? Change everything at will letting your ears tell you what's right. You might want to put on a drum beat to play to. It will help your timing.
It's real easy to overthink this, so don't fall into that trap. Keep things simple and experiment. Things will work themselves out. And don't be afraid to ask "hey what key are we in?" It's a great skill to be able to play by ear, but it's not easy. Also, some people have great memories and can memorize the various keys, others like me stick to a few and if I get stuck I just ask someone what's going on and go from there.
The people I play with range from a professional guitar player who is expected to know all sorts of stuff, and he is a great teacher, kind of like a Rick Beato character, to guys like me who just want to jam and all kinds of stuff is flying around the room. But we share and help each other. I just learned how to play Tangerine this morning! Didn't know that was coming, but that is why it's so great to find people you can jam with.
Good luck.
edited
This year the diet is definitely gonna stick!
