As you are taking piano lessons, your teacher will help you with the basics. Learning to read sheet music is easy! Especially for piano playing, as the layout on a piano is extremely user friendly. So you learn the basic principles, and the rest will follow automatically.
As you are going to buy a digital, make sure that you buy one with weighted keys so it is alike acoustic pianos. Otherwise you will not learn the right piano technique properly.
An important feature that comes with modern digitals is the recording feature - use it often, that is, LISTEN to yourself in recordings a lot, that will be really helpful. Because you don't just have to train your hands, but also your ears ...
I also think it is very important to learn the right posture and movements from the very beginning. Far too often I see beginners who have terrible, tense movements that will give them pains and injuries for sure, and their playing sound accordingly ... So let your teacher guide you and try to find a correct posture which allows you to play with a minimum of effort, that will save you from a lot of problems later - and make your playing much better as well. It is a good investment to start with this. You don't have to rush theory learning; let it come in small portions.
My last advice is this: don't go to the piano and start to play unless you have a very clear plan on what you want to do this time. Personally I write a practice journal where I jot down notes on what I intent to work with - small sections of pieces, and HOW I intend to work with them (very slowly, hands separated or something like that). Setting up vague goals like "learning to play Für Elise" is NOT a clear plan.
Once you have learned to read sheet music - and again, you will learn rather quickly - you can make it a good habit of studying the music BEFORE you go to the piano. "Read" the piece carefully a lot of times and resist the temptation of trying to play it at the same time. When you have got more used to piano playing, you will even be able to imagine how you play. No, you don't have to have perfect pitch or memorize the piece perfectly, this is just a preparation that will speed up your learning and effectiveness quite a lot. So when you finally sit down at the piano, you will know exactly what to do and the piece will already be a bit familiar to you.
I really wish someone had taught me this way of learning from the very beginning.
Topic: Tips for a beginner