Starting with what you've got is a great idea. Start with whatever you already know and are comfortable with, and then you can build from there. So you can try coming up with a tune to play on guitar, and then try writing the notes down accurately using free software like Musescore. Or, the reverse, try writing something for guitar using Musescore, and then try playing it. Either will help you get better at knowing when something is a half note, or a quarter note, or a dotted eighth note. Either will help you get better at reading and writing pitches and knowing when to use accidentals, and when the sharps and flats are part of the key signature.
I like the advice to start with children's songs or folk music. You could also add Christmas music to that list, or songs from summer camp, or any music that you know by heart. Anything you are very familiar and comfortable with is a good place to start. Do one or two of those, and then when you feel like you can accurately write down the notes and rhythms you want and know the basics of the composing software you are using, you can start arranging pieces, or writing your own tunes. It's like learning to read and write. We start with picture books, so we know what is going on, even if we get stuck over the reading. We start with being read to by someone else, and following along with our finger on the page, and writing letters with a pencil. We start writing short, simple sentences. Then we write short stories or poems or book reports, and can read more fluently, and we learn to use Microsoft word and type quickly, and then can write whole novels if we like.
But for now, don't overthink it, just get started. 🙂