Root, grow then blossom.
Believe it or not, the most common issue most piano learners have is quite simply: "What should I do first?" followed by: "OK, then what?" - In a nutshell, one must establish roots so that one may grow firmly so that one may blossom fully.
This is how Nature works. You are Nature. Do what Nature does.
Allow me to expand on this.
There are many species of tree but even within one same species, no two trees are exactly alike; a difference in branch layout, height, number of leaves, etc. This is to say that, although you are all humans reading this, you are all also so very, very different... and thank goodness for that!
What is the same for all trees, however, is that no matter the beautiful end result, they all have roots and would not exist without them. Their roots gift them the opportunity to grow tall, stand strong in gales, take warm sunlight, give pure oxygen, soak up life water, provide a safe-haven for various wildlife and even teach us a thing or two if we are quiet enough in our minds to pay attention.
You see, what is the root to the tree is the major scales to the pianist. Without them, pieces are impossible to comprehend and acquire, chords remain undiscoverable, the fingers have little to use to practise precision and dexterity, note value awareness is baseless, progressions are impossible to build up, other scales are more difficult to identify, modal theory is rendered unobtainable, sight-reading is more challenging, ... should I go on?
No matter which path you wish to take as a pianist, from a stay-at-home hobbyist playing three-chord pop songs to a world-touring concert pianist bashing out the great, demanding compositions of the 19th Century, without absolute mastery over and awareness of those twelve major scales as your firm root, you are as good as a dried-up pile of twigs. I really, really mean that.
I have already written on how to use major scales and make them more interesting and I have created a video for you to combine the major chord types with major scales in order to better memorise them so I shall not repeat such topics in this article.
Instead, I would like simply to encourage you to solidify your roots so that you can make better, more enjoyable progress with the path you wish to take. This is "What you should do first", to quote the opening paragraph.
"Ok, then what?"
Prioritise and master the smaller components of your path.
1. A solo pianist working in a restaurant, on a cruise ship or for functions will need to acquire around 500 pieces, primarily from the 1900s to the present day, including a few of the more famous Classical pieces such as Moonlight Sonata by Beethoven or Beautiful Blue Danube by Strauss. Get to work, once having established your firm roots, on acquiring repertoire and discovering how you play pieces naturally. You will have to have basic right-hand melody sight-reading skills and the ability to understand chords. Progress alphabetically by song title or go decade by decade taking 15-20 favourites from each one.
2. A piano teacher would not be required to have a large repertoire but very good sight-reading skills, a wealth of theory and a good understanding of hand and arm anatomy to avoid inducing damage. They have spent a lot of time listening to many recordings of pieces and have a very good ear to detail, understand piece interpretation and encourage self-discovery. Progress by acquiring Grade 8 and a Teaching Diploma.
3. A concert pianist has primarily a 19th Century repertoire but of course this is not to reject the 20th. An inordinate amount of time is spent on technical exercises, absolute piece mastery, sight-reading, memory enhancement, precision in touch and leaps, listening to other performers, possibly entering competitions (or judging them) and, if they are of a moral standing, promoting lesser-known works and composers from all eras. Progress by acquiring a perfected and tight but not necessarily excessively large repertoire, establish strict habits in terms of technical exercises and piece memorisation and never, ever stop to think "I have made it".
4. A composer does not care so much about acquiring repertoire and usually only advances their technique to fit the pieces they compose but of course do always try to push your technique further so that your pieces can better reflect your emotions, not restrict them. Much time is spent on observation, reflection, self-analysis and listening to melodies which appear spontaneously in the mind. Write them down or record them. Progress simply by spending some time at the piano when there are no distractions and allowing your fingers and Inspirational Source to work together, without your ego getting involved, and see what happens. You will be pleasantly surprised.
No matter which way you grow and eventually blossom, know that if it is honest, Purposeful and the product of a balanced and patient mind, success is inevitable.
I've planted the seed, it's now down to You to water it.
What will you become?