Advice for beginners - practice!
If you are learning an instrument it is important that you set aside time to practice in between each lesson, this will enable you to get the most out of the time you have with your teacher. Ideally I spend the lesson time warming up, going briefly over what we covered last time and building on this to cover new techniques and concepts and perhaps starting new pieces. However, if you have not practiced beforehand, much of the lesson will be taken up with practising so that you are ready to move on to new things.
I recommend different amounts of practice per week depending on your age, level of experience and goals. Many new students pressure themselves to practice for long periods at a time, but I have found setting unachievable goals actually stresses students out more and means that they practice less! As with many things it is quality not quantity that counts, and so frequent, short bursts of focused practice (e.g. 20 mins x 5 times a week) are much more productive than a long slog once a week, the night before your lesson, once you have forgotten everything from last week!
Understanding how to practice and what to practice is also very important and it is something that I cover regularly during my lessons, often writing down notes for students to refer back to during the week. I am also happy to spend a few minutes at the end of each lesson describing to parents what we have done so that they can better support their child's practice.