Buy a camera equipment bag that will hold and protect the equipment you most commonly use. This statement may seem to be little more than common sense, but it is good advice since many people purchase more or less in a camera bag than they actually need. But, if this is your first bag, don't make the mistake of buying too large a bag that you can't lift and carry comfortably, and don't get too small a bag that will be impractical. And consider the likelihood of your purchasing an extra lens or some new filters in the future. Be sure the strap is sturdy and wide where it rests on your shoulder.
Side pockets are a useful addition.
Plastic or metal camera cases are more resistant to abuse, but unless your camera bag will be on the bottom of a luggage pile, they are usually less practical than soft cases. You take a chance in buying a camera bag by mail order because you don't get to handle it beforehand to check that it will hold all your gear, try its weight on your shoulder, test its zipper and snaps and so on. If the bag looks used but is still serviceable, thieves might think it contains older, less-valuable camera gear and pass on it. Examine all of a bag's features to ensure they suit you. Most photographers find two bags more practical than one large one.