Hello, I am a sixteen year old guy and I practise piano by myself. I already know 3 pieces and all scales and arpeggios. I would like to know what piece I should undertake right now.
Pieces that I play effectively : Nocturne in C# minor, Nocturne in Eb major and Turkish March
PS: I really enjoy pieces from the romantic and classical period.

I appreciate any help already provided.

Thank you.

Welcome to the forum, Victor.
Choosing a repertoire is always a difficult task even for professional concert pianists. It depends on many, many factors such as the type of event, the level of abilities, availability of time etc. I think it's impossible for anyone that doesn't know you or heard you even once to suggest a suitable program for you. Choosing a program is something one learns with time and experience.

Victor, welcome to the forum! It is very hard to recommend music without hearing how well you play your Chopin and Mozart. It's impossible to tell what you mean when you say you know all scales and arpeggios. How well do you play them? How even is your articulation? How fast? etc.

Are these the only 3 pieces you have played, ever? Do you have a teacher?

The Adult Beginner's Forum might be a better place to ask.

Some ideas that come to mind: Beethoven's "Fur Elise" or the first movement of Sonata Opus 27 #2, also known as "The Moonlight".

I practise by myself. I practice with the metronome. Before I started learning pieces I started playing arpeggios and scales in all keys. I can play them in 4/4 tempo, eighth notes. All three pieces are well done.