There are many golf bags available, some designed for fashion, some for utility and some that combine both features. There are big bags, bigger bags and bags so big that they would keep everything I need for a long weekend vacation in one of the side pockets!
Generally speaking, you need a bag just large enough to hold your clubs, extra balls, your glove, tees, car keys, extra pencils, ball markers, a ball retriever, sunscreen, a windbreaker and a large umbrella.
It is also a good idea to have a packet of tissues, a band-aid or two, and if you play courses where insects are a problem, a can of bug repellent comes in very handy.
A small pack of baby wipes come in very handy; in your bag they get warmed by the heat, so when you get sweaty or a sand trap covered you with sand, a nice warm wipe can be very refreshing.
I happen to be allergic to bees, hornets and wasps, so my Epipen is an important addition to my bag. I am also hypoglycemic so I carry Lifesavers, which has to be replaced several times throughout the year because they tend to melt.
One item that doesn’t need to be in your golf bag is your cell phone. If you must carry it, turn off the ringer as a courtesy to other players. If you must use it, be aware of others who might be taking a swing or putting their ball. Be considerate!
Having these items in your golf bag should provide everything you will need, even in a minor emergency (like a blister) or a major inconvenience, like a sudden rainstorm. Being prepared makes the game a great deal more fun.