Its important to know a few heads when you show up for a jam. Those you can get out of the Real Book. Close enough. Listen to a few versions to find a few embellishments on the head you like. If its a vocal tune listen to singers play it. Ella, Frank, etc. Tony Bennett is something on a lot of Jazz tunes.

One exercise I use is playing the 4 note chord arpeggios across the changes. 1,3,5,7 are the tones you are looking for.
Adjust major,minor,diminshed,dominant as necessary. For the time being dont worry too much about the 9ths,11ths 13ths.
You'll get there
As you advance You can add passing tones (diatonic and/or chromatic) to them to spice it up.
Hammer ons and triplets add a nice rhythms to phrases.
When going from chord to chord try landing on the 3rd of the chord. Pivot arpeggios will help you on this.
Passing tones help here also

Listen to the players you like. Not trying to imitate them, just trying to get a sense of their phrasing.
Knowing what to play is important as is Knowing how to play it,. The true lifelong journey.

Above all Be brave, don't worry about a clam here and there.
Play with others everytime you can.
Finally, If you're the best in the room, you're in the wrong room

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