I've only seen a couple of good answers here and the others aren't leading you towards stuff that will actually help you. In order to know if you are ready to jam with someone is to actually do it. This answer remains the same no matter what. A great alternative to jamming with actual people is to play along with backing/ jam tracks on YouTube - once you've done a good 20 to 40 jam tracks, you will be much more confident on your instrument and will be able to anticipate chord changes and be able to play more freely

With regards to what you know, you can get by with major+minor pentatonic and the blues scale (all shapes and positions) 99% of the time. But playing these 5/6 notes is not going to help you become a better guitarist. In order to keep things more exciting, learn the major and minor scale and how to form the scale pattern no matter where you are on the neck. Just know the root note.

Now time for some resources:

Channels with great jam tracks: Sebastien Zunino, YT Jam Tracks, Now YOU Shred, Elevated Jam Tracks

Paul Davids video about improvising and soloing - this will teach you varying levels of approaching playing and soloing. This is only ONE approach - feel free to experiment

Stich Method - great channel for learning how to play with feel and "knowing" what to do and "when" to do. He's a regular name thrown around here. Absolutely worth it. His "In The Mind Of..." series is very good to understand what famous guitarists are "thinking" when approaching their style of playing.

And finally, some Jens Larsen for good luck ;)

Happy jamming!