A mistake many new students make is thinking that they will be playing their favorite songs on the very first lesson or shortly thereafter. I hear this all the time. Remember when you first learned to throw a ball and how much effort it took to become accurate and always hit the target? It took time, patience, and perseverance to accomplish. Learning to play an instrument properly does too. It also needs to be cultivated to a level of accuracy.
Understanding that learning to play an instrument properly does not happen over night, and is a process of training the body and the mind to work seamlessly together so the music will be a wonderful result, will make learning much more enjoyable. That said, if you follow your instructors advice you will get to those things you want to play faster than you might expect, and experience an achievement unlike anything you could imagine. All of us were students once too, and do understand.
Another one I hear a great deal is “I only want to play this kind of music.” The learning process applies to all kinds of music. When you learn the language of music you are opening up the doors to everything and anything you might ever want to play. It is the training and the knowledge you gain that allows you to pick and choose at a later time.
New students should expect to make gradual achievements, mastering one small goal after another. This will eventually bring you to playing things you want to play while allowing for your musical tastes to change as you learn more. In other words, you are always opening doors for a new future musical experience. This is what a proper musical education is designed to do and will when you apply yourself.