Stock Your Bag
Diapers (at least one more than you think you'll need)
Hand sanitizing gel
A water bottle (for you, especially if you are breastfeeding)
Burp cloth
Change of clothes
Stain remover (wipes are convenient)
Pacifier (if your baby uses one)
One small, favorite plaything (no need to pack the entire contents of the toy box)
Baby sunblock (if it's sunny out)
...that way, all you have to do is toss in bottles (if you aren't breastfeeding), your wallet, cell phone, and car keys. Bye, bye!

Essentials for On the Go

And away we go! It's hard to believe that transporting a baby can require so much paraphernalia. But as much as you might think it would be just as easy to carry your child in your arms everywhere, that's not exactly a practical or safe option. Here's what you need to get from Point A to Point B with a tyke in tow.

A sling or front carrier will allow you to "wear" your baby on short walks or even while you go about your daily business. Either one should be washable, feature padded shoulder straps (for your comfort), and be adjustable. Front carriers should provide support for your baby's head; it's a plus if it can be used when she's old enough to face forward.
The options are endless. To get you started, keep these features in mind: sturdy construction, rugged wheels (air-filled tires are good if you'll be covering bumpy terrain; so are shock absorbers), storage space, lightweight, easy to fold (with one hand is ideal), and compatible with your baby's car seat (or consider a travel system--stroller and car seat sold together). The stroller should also be safe and comfy for your child: amply padded, with an adjustable seat back, a hood or canopy to protect him from sun, wind, and rain, and a three-point seat belt.
This purchase is a biggie, but not as baffling as you might think. Just keep three things in mind: the age and weight of your baby (she should sit rear-facing for as long as possible); your car (if it's a 2003 model or newer, it will be compatible with the LATCH system that's required on all car seats--so installing it will be a cinch); and the car seat's safety restraint system (a five-point harness is the only way to go).
There's a style out there for every taste (including bags made just for dads!). Looks aren't everything, though: The best diaper bags are roomy inside without being bulky; have plenty of compartments inside and out--for bottles, pacifiers, small toys, your stuff; are washable (if you can't throw it in with the rest of the laundry, you should at least be able to wipe it down); and easy to carry (backpacks are great, as are bags designed to hook on the back of the stroller).
Most diaper bags come with one--but we wholeheartedly advise you to leave it at home (use it for a coaster under your houseplants, maybe) and outfit your diaper bag with a bigger, better one, a pad that's longer and wider than your baby, so he'll have full coverage underneath him; easy to wipe down; and, as a bonus, designed with pockets for a few supplies. That way you won't have to lug your entire diaper bag on short errands: You can just bring the pad with a diaper and wipes inside it.