You select the cover based on the type of guitar you own, classical, acoustic, or electric, all of which come in a variety of sizes. You can use soft coverings with a handle if there is little chance of striking the instrument during transit (more by vehicle) or waterproof materials with a foam liner if you walk.
If you are carrying additional materials, you may select covers with big pockets for scores, a support for securing the faucet in order to maintain a steady instrument position, and specific compartments for accessories.
You may choose from simple versions with a single handle for carrying in the hand, shoulder straps that must be adjustable for a good fit, or models with backpack straps for carrying in the back. A hard plastic guitar case, which provides superior protection for the instrument, is also available at a greater price.