From Blog:

“Many people primarily play sheet music or play along with their favorite songs. Eventually this gets boring, like listening to the same song on repeat for too long. There is no spontaneity and it is at this stage where people lose interest in playing an instrument.” http://blog.jammr.net/2013/07/28/why-every-musician-should-learn-to-jam/

That's right. Jamming is the next step level. I learned some music in elementary school as a part of general education many decades ago and at that time rock was not an issue for schools. I learned especially a bit of diatonic scales and some folk songs. After a long breakdown I started again playing guitar and other new instruments. I must play some “boring songs” to build the basic skills first before jamming with you. Only changing chords is difficult test during jams. I found on the Internet some American folk songs (about 10 songs for ukulele, Somewhere Over the Rainbow among them) but i don't know what to practice to get an approach to jams. Can you suggest a must do list of songs for preliminary practice?

adi
I found on the Internet some American folk songs (about 10 songs for ukulele, Somewhere Over the Rainbow among them) but i don't know what to practice to get an approach to jams. Can you suggest a must do list of songs for preliminary practice?

Chord changes are important. When you practice a song by yourself, also make sure to use a backing track or metronome. In a jam there will be a beat that everyone follows. When playing alone without a metronome, it's easy to miss the crucial exercise of playing to a tempo.

Learning songs in different styles of music also helps. You never know who else will be in a public jam. If you are familiar with different techniques from blues, jazz, rock, funk, etc you will be more confident. With techniques from different styles under your belt you can jam more easily with other people - they may not play the one style you play the most.

After you are fluent in the basics of your instrument (chords, sticking to a tempo, scales) and techniques for styles, the rest is about getting experience through jamming. You'll find out what sounds good and learn tricks from others as you play.

I don't have a list of songs that will broaden your ukulele playing, but you could start on the Wikipedia page and look at famous ukulele artists:
https://en.wikipedia.org/wiki/Ukulele

Stefan

when ukulele playing comes to mind i can only think of one person George Formby

http://www.ultimate-guitar.com/tabs/george_formby_tabs.htm?no_takeover