Here are a few things that should be in your newborn baby diaper bag:
1. Diapers, but of course. Plenty of them, you'll run out quickly. I'd say at least 10 diapers is good.
2. Wipes- We got a thin plastic case to put wipes in. We also leave a wipes case in our cars in the event that the smaller case runs out.
3. A few books, rattles. You won't need these until a few more months, but it never hurts to toss them in there.
4. Toiletries- I put in a travel size of body lotion, some diaper rash cream and vaseline, etc.
5. A travel size changing pad- We got a great plastic material one that folded up, it's black and was cheap at Babies-R-Us. You'll need this for sure. A must-have.
6. Clothes- At least 5 sets of clothes (when they're newborns anyway, you'll need these, as they have accidents often!). Make sure you have a couple pairs of socks, one hat, at least 5 onesies, maybe 3 pairs of pants and shirts sets, and at least one sweatshirt or something warmer.
7. Disposable items- My boss who has twin boys gave me some great gifts, disposable bibs and plastic bags to put dirty diapers in. These were great!
8. Bibs and burp cloths- At least 2 bibs and at least 3 burp cloths. These come in handy for spit ups, drooling, etc.
I think that should cover it. The items vary when the baby gets bigger, so these are must-haves for newborns. The biggest thing to remember with a newborn and the diaper bag is that newborns change frequently - their clothes and their minds - so you need to be prepared for anything at all times. Having what you need, organized in the bag will make your life easier in those moments when you need to make a quick change.