You need to spend some money on a camera bag (sorry to tell you), for convenience, and to protect your precious gear.
Where to start?First you need to know if you are looking for a bag to go on a trip with, or a bag for any photo walk. In other words, do you want a bag to carry all (or almost all) your gear or a bag to carry a selection, small or not, of your equipment (the selection may change depending on many things: goals, weather, time of day, ...).
Having both types of bags may be a good idea.
You know you will at least have your dSLR and memory cards. Then do you want to carry all your lenses (if not, which ones?), flash unit(s), tripod, etc. For now just consider the biggest equipment you have, you need to know what bag size you are looking for.
AccessibilityCamera bags are supposed to be designed to allow you to get your stuff as quickly as possible. You don't want to miss a photo because you couldn't get your camera on time. Same goes for other stuff you might want to be easy to access, such as memory cards, batteries, or your second favorite lens.
Most bags include modular systems to help you make the best of the available space.
ComfortShoulder bag, rolling bag, backpack, holster bag, all kinds of bags exist. I have a single strap backpack (+ small side strap), it is very handy, you can pull your camera in the blink of an eye, but I confess it is hard on the shoulder after a few hours.
PricePrice should obviously be taken into account in your decision, but don't buy cheap if you think you will regret it. You will have your bag with you all the time, it is an important purchase.
My best advice is: try them. Take some of your stuff and go to the closest shop to try the bags, test all the points that might help you make up your mind.
Famous brands include Vanguard, Lowepro, Think Tank, Tamrac, Crumpler.