Learning about scales or keys, especially when you are a beginner can be rather confusing and daunting. I’m not surprised that it is daunting because there is so much information on the internet and lots of people with lots of opinions about what is essential to know. It can get confusing, especially when there are people voicing an opinion, that really don’t know enough to be telling others the way it should happen.
Ultimately, if you are planning on being highly skilled, then learning about lots of different types of scales is important. This is especially important if you intend to play jazz piano and want to be able to improvise because the more knowledge that you have, the more tools you have with which you can improvise with. If you are interested in classical piano, then knowledge of a large variety of scales can help you understand and learn complex classical pieces. If playing pop/modern music, then the basic major and minor scales will probably be enough information, however, if you are interested in learning more, then certainly go ahead and learn more.
If you are just starting to learn to play the piano, no matter what genre or genres you end up playing, C major scale is the first scale that you should learn. Every beginners piano book I have ever looked at, teaches C major first. C major is the best scale to learn first because it is very easy to remember because there are no black notes used at all, or to put it in a technical way – there are no sharps or flats in the key of C major. After this, I would recommend learning G major (that has one sharp – F sharp) and F major (that has one flat – B flat). Once these are thoroughly known and understood, A minor would be the next scale or key to learn. It is important at this stage to not only know what notes are played but also how and why it is related to C major.
Once you have this knowledge, then it is relatively straightforward to learn slowly over time, all the other major and minor keys or scales. For reference, it takes on average about 5 or 6 years to learn them all, so if you don’t know them in a year or two, don’t be worried about it.
During this period of time, it is also useful to learn what the difference is between an harmonic minor scale (which is often just referred to as ‘minor’), a melodic minor scale and a natural minor scale. The differences are fairly easy to learn, but only when you have a good foundational knowledge (which I mentioned earlier).
It is only when you get to an intermediate or advanced level of playing the piano in whatever genre you prefer, that you need to learn about pentatonic scales, whole tone scales or modes. You could, if you were interested, learn about other scales that belong to other non-western musical traditions. Personally, while it might add more interest in your piano playing, especially if you were into improvising, I think learning about scales from other non-western traditions is best left for when you would be considered at an advanced level of playing the piano.
So if you are just starting out on your journey of learning to play the piano, please don’t get overwhelmed with all the different types of scales. Just start with C major and slowly take it from there.