Put aside anxieties about "composing correctly" and just start writing down your music. You can begin to refine your writing skill as you gain experience composing.
https://www.youngcomposers.com/
When in doubt, arpeggios can sound great on their own - a lot of Einaudis pieces are mainly just arpeggios and they sound great, like Divenire, and then there's Beethovens Moonlight Sonata's first movement. If you can't think of a lovely melody then don't give up, the chords themselves can be the tune.And don't worry about formality and theoretical "correctness". Form is simply a way of organising your ideas, Sonata form, ternary form whatever - it's there to help not hold you back. If you know what you want to do next with your piece, do it. If you're looking for ideas on how to proceed, then if you look up how a Sonata works you might be able to get some inspiration.Same with harmony, don't spend ages looking up how Classical harmony is supposed to work and obsess over keys etc. it's as Liszt said: "Any chord can follow any other chord" If it sounds right it is right.