I don't think I could even do one sit up. Where do I start with strengthening it?
I'd start by thinking about your body alignment & posture. To have decent skeletal alignment & good healthy posture, you need to use your abdominal muscles - they operate like a kind of corset around our middle.
So think "navel to backbone" or the Pilates "zip up" all the time - get used to walking a sitting at least 50% of your time in a "held" posture with a straight spine & neutral pelvis. Suck it all in - although keep your pelvis soft & neutral, don't flare your ribs or tens your shoulders - you need to use your abdominals to do this.
This will start to activate those support muscles, and show you the importance of functional fitness - abdominal muscles are there to support your back. There's no point in doing sit ups just to do them - think about why or what for you need strong abdominals.
And if you do this in conjunction with a good Pilates class, you'll start to see some results. But don't go madly crunching. Half a dozen slow, controlled perfect roll ups and spine roll downs, with no bunched up stomach, will do you more good than umpteen sit ups.