Open mic jams can be a good place to meet musicians and practice jamming.
Some open jams are sort of "pro," where most everyone is experienced/high level, and you probably don't want to do one of those. But many (most?) are not, and feature all levels, including beginners.
Find one of those, go hang in the audience a few times. See what people are playing. See how it works. Gain inspiration by seeing other beginners get up there, and see how supportive the audience is. Talk to people.
At a jam, everyone in the audience is a musician. Some of them will be just as nervous as you. Tell them you're new, inexperienced, want to jam, but are nervous. You will get lots of encouragement to get up there. And lots of support if/when you do get up there. Open mics are good for that because many folks in the audience aren't confident about their playing either, so knowing that, we all support each other and make it a safe place to learn.
Another idea: try Craigslist. Be perfectly straight about your level and goals. "Hey I'm a newbie, very inexperienced, looking for somebody to practice jamming with." or whatever. You might find a music partner for life.
Best of luck!