I want to show Incredibles 2 to my girlfriend, but she hasn't seen the first one.
Does she need to see it first? Or maybe, what information from the first part is necessary to understand the second?
While the second Incredibles can legitimately stand on its own , I would think that the viewing experience would be diminished if one had not seen the original.
The sequel picks up just as we left the intrepid family and the transition carries one of the character development stories along with it. If fact, this might be one of the character development subplots that your 'gf' would be interested in.
There is a subplot starting in the original involving the daughter's growth through adolescence and overcoming shyness towards boys. Due to the 'super' adventures the family experiences on the island and back in the city, Violet Parr gains self-confidence and accepts/arranges a date with one of the boys from her school at the end of the first movie. In the beginning of the sequel, the same boy sees her without her 'disguise' (mask) and realizes who she is. His mind is subsequently erased by government agents responsible for maintaining 'super' secret identities. Along with the secret identity, he also forgets he has a date with Violet and even who she is 'cause, y'know, mind erasure isn't an exact science. This leads to a teenage temper tantrum and other trouble in the Parr family.
While this plot point is sort of adequately explained through narrative and short flashbacks in the second film, experiencing and identifying with the character's angst and anxiety through the whole situation from start to finish may be one of reasons that your 'gf' ends up liking the franchise.
The above is just one of the stories that are carried through from the original to the sequel so ultimately, I'd vote No; see the first then the second. Put together back-to-back, they are still shorter than Lawrence of Arabia.
Since we know that The Incredibles 2 will pick up immediately from where the first movie left off, what happens in The Incredibles is actually quite important to the sequel.
The most important thing is we learn that superheroes were effectively outlawed by the government after a bunch of normal people began suing them for damages.
So they're peacefully living their 'normal' lives, until it's time to leave - they head to the parking lot, and suddenly the Underminer is there with an oversized drill proclaiming
"I am always beneath you, but nothing is beneath me."
So it's time to suit up and get to (still illegal) work. And that's exactly where we find them at the start of The Incredibles 2.
My recommendation is to see The Incredibles before watching The Incredibles 2, but it's not required to understand the plot of The Incredibles 2.