You aren't going to jump right into a great band. There are plenty of bands for people like you who are still figuring most things out. If you want to play with others I'd suggest getting your girlfriend to help you a bit more and then make a Craigslist ad saying you have never played in a band and are looking for other beginners to jam with
This was my thought.
And start really simple. I mean SIMPLE simple. Have your GF just strum a G chord or something and you just pump out G. No chord changes, no other notes. Just play that G till you can feel you're in a groove with each other. Then go to another chord, a D or C or something. Pump that 2nd chord forever until you're feeling the groove - and then you can start alternating a bar of G, bar of D, bar of G, etc. From there you can go on to super simple songs - "Good" by Better than Ezra, "With or Without You" by U2. Take your time and work into it slow and easy. Don't worry about doing anything more complicated till that gets boring and you're itching for more.
Then, yeah - craigslist ad, "beginning bassist looking to jam." Take it from there.
If you make a mistake, doesn't matter - the instant you make it, it's already gone and in the past. Let it go. Play in the now.