It is good to get your beginner piano music in one place. That being said, having several choices to get things in one place could be a lot better.
If you're looking for beginner piano music do yourself a favor. Watch out for and be careful when visiting some of the giant sites that appear to have all the music in the world in one place. You could get distracted and waste so much time online looking and searching, and mostly coming up with nothing. Worse still, you might end up with something you don't need, or can't use. Been there done that.
Whether you are a fresh newbie, been at it for a while, or polishing up the beginner level, going with a good method book could be in your best interest. It's a very good way to learn how to play the piano. If you know how to practice and use the written music properly it will prepare you for higher levels of piano playing. Very often, I've taken students through a beginner method piano book and moved them to the Royal Conservatory graded repertoire series as soon as they felt ready for it.
There are several good beginner piano music books out there. I've made a carousel of a few of my favorites with a brief description of the features and benefits of each book or series. Here we go.
Some Beginner Piano Music Books
Alfred Adult Piano Course: Is very good with showing the proper sitting and hand position, basic keyboard to notes layout, and includes some well known tunes like Greensleeves, Mary Ann, When The Saints Go Marching In, and others. Be sure to check the online table of contents before you buy if you have a favorite tune that must be included in the book to make you happy.
Explanations are clear and easy to understand as well in this series. I've never had a problem with any of my adult students using the Alfred series of beginner piano music books.
The book of scales and chords in the Alfred series breaks things down and gives just the right amount of keyboard depictions to musical notation to make it understandable. Still, people tell me that they wouldn't move as quickly or as thoroughly without a teacher. Scales and chords could get tedious without a helpful nudge from a good teacher.
John Thompson's Modern Course For The Piano: At first glance doesn't look very modern. Still, with an actual photograph of good hand position and clear break downs of musical patterns used in the method make this book almost foolproof.
I have frequently used this course with students of all ages. I find it takes solid steps with clarity and is easy to read.
150 Of The Most Beautiful Songs Ever: I love this book. It is not a method book and does not contain lessons. I had to include it here as so many of the tunes I play, especially at parties and events, come from this book. It really is not for new beginners. However, if you learn to play at the grade 4 level you could do many of the songs in this book. If you get to a grade 8 level of playing ability you could do most or all of the arrangements under its covers.
Summary
If you start, just start, you will move forward. If you just think about it, and think about it, that's what you'll get. Thinking without learning and playing.
So, good luck. Let me know if I can help you get going. Small steps will eventually get you there. Good luck!