Of all the things a new parent needs, a diaper bag is a must right from the start. Babies require so much stuff! Even for short trips, you need diapers and wipes, a bottle or two (or a breast, depending), a burp cloth (there will be messes and you will need this), a change of clothes (diaper leaks happen alot more than you might expect), and any medication.
The hard part about being a new parent is that, at first, you have no idea what you need. Diaper bags are one of the hardest things to pick out! There are hundreds out there, ranging from quite inexpensive ($20) to extraordinarily, outrageously expensive (hundreds of dollars). A diaper bag is not special in that it is not going to magically produce diapers or be a Mary Poppins bag magically producing a spoonfull of sugar. It is just a bag. But as a new parent, and maybe even an experienced parent, you are told by every outlet that you must spend a fortune on designer bags but, in reality, these bags pretty much just suck. They are not usefully designed! They aren't large enough for a full day out, they don't have appropriate depth in the main compartment or pockets with depth or comfortable straps. Do not be fooled by a bag that LOOKS good. My first large diaper bag was not cheap but it is, sadly, quite useless. Nothing fits in it and the strap is downright painful. For some reason, it is really padded and quite heavy. I searched and searched through hundreds of bags, both at the stores and online, and have finally found one diaper bag I can recommend strongly. And the best part? It only cost $20!
The Fisher Price 3-in-1 Doctor diaper bag is fabulous! It is 5-6 inches deep (front to back) in the main compartment so there is plenty of room for a stack of diapers, change of clothes (or two), plenty of bottles (I carry 6 9 oz bottles), formula dispenser (more on that later), small toys, medication, dirty-diaper bags, and just about anything else you will need. It opens very wide so you can easily see everything and pull out only what you need. There are mesh pockets with elastic along each side so they lie flat if not in use but are great for holding diapers and such. The two front pockets are 1 inch deep and zipper at the top. One is padded. There are two flat pockets on the front of those that are great for holding a pacifier or sunglasses. The sides of the main compartment also have flat pockets that hold hand sanitizer, tissues, etc. The strap is wide and padded plus there are short carry handles so you don't have to always use the strap (not all diaper bags have this and it is a must). The bag itself is made of sturdy material that is easy to wipe clean but it is not so thick that the bag is heavy by itself, as alot of diaper bags are.
I also purchased the Jeep bag that is made very similar, the Jeep Clamshell. It is not as well designed as the Fisher Price bag. There are no interior mesh pockets so everything just jumbles together. The material isn't as sturdy either. Otherwise, it's a good bag. It's about the same size as the Fisher Price bag so it has plenty of room in it. I really like it best for an overnight stay at grandma's since plenty of necessities plus extras fit into it.
Also purchase a small diaper bag for quick runs to the store. A couple of diapers, wipes, change of clothes, and a bottle is all that needs to fit. Have a little extra space for mom's wallet and that's all you need to carry.
The items that go into a diaper bag seem common sense. Of course you need diapers and wipes and bottles. It's the little things that I didn't know about initially that have come in really handy. First, if you are unable to breastfeed and use powdered formula, the Munchkin formula dispensers are great. They are a 3 compartment item with a rotating lid so you can put as much powder as is needed for one bottle in each compartment. Fill the bottle with the appropriate amount of water and everything is easy to make when out and about. I have not had trouble with them leaking or mixing the measured contents. Just be sure to align the lock correctly.
Some other items I didn't think about needing were small hand sanitizer, a 3 ml medication dropper, dirty diaper bags, and a paci bag. The 3 ml medication dropper is surprisingly difficult to find. If baby has reflux and requires 1.3 ml of medication, using a syringe is inconvenient. The medication dropper is easier, especially at 5 am when no one is awake except screaming, hungry baby. I ended up buying one from Jeffers Pet because it was the only one I could find that had enough graduations to reasonably measure out the medication.
As for dirty diaper bags, when you're out, you don't want to leave a dirty diaper in someone's trash to stink up the place. Dirty diaper bags are exactly like doggie poop bags. In fact, if you buy the doggie poop bags and dispenser, it's about half the price of the same thing at Buy Buy Baby. I found the doggie ones to be sturdier. I like the fire hydrant.
Pacifier bags are actually much more useful than I thought they would be. When baby is screaming in a store and you are frantically looking for paci, it's much easier if it's in its own little bag attached to the diaper bag. The paci bags at Babies R Us and Buy Buy Baby are way more expensive than is necessary for a piece of cloth and a zipper. The Kmart paci bag is $2.99 and has a velcro strap so it's easy on and off.
While on the diapering subject, dirty diaper bags also come in quite handy at home. You don't need to purchase a diaper genie, and if you're doing cloth diapers then there really is no reason for one. Use the dirty diaper bags as the diaper, wipe, and/or diaper liner receptacle. Tie it off and put in trash. The smell will be way reduced. A diaper genie basically does the same thing for much more cost and trouble of buying the correct bags for the particular can. Money saved on things like this is money that can be spent on more important things.
