Great post! There are endless ways of writing songs. I try to challenge myself to always write at least 1 song differently every month.
That being said, if your starting out, there are two ways that make it easier to write I'll mention.

First way, is to take an already existing song you love, and find an instrumental/production version of that song online. Play it on repeat, and write your own song, to that production.

Second way is to record yourself playing an instrumental chord progression and loop it, or use garagand or any DAWS stock loops, and make a looped production. play it cycled and write a song to it

These methods are easier because you can focus on writing melody and lyrics, instead of playing the instrument too. As for the song itself, some people focus on melody, others on lyrics, others do both at once. For instance, I usually write melody then put lyrics to it, or I do both simultaneously. I almost NEVER write lyrics first and then make a melody, I've tried but for me its harder. Everyone has a different workflow, its all about making something you love, being happy/having fun, and finding your most efficient workflow.

Don't worry about accidentally stealing a melody. It happens all the time. If you or someone realize it, you can always change the melody, or lyrics, or chord progression, or tempo, or production etc.
Also at the pro level, I have friends (not me) who have written big songs, and they actually buy a special type of creative insurance, that will cover lawsuits up to Millions of dollars (crazy right?) incase you get sued by someone. You don't have to worry about that until you write a hit song that goes national or viral, or write for a famous artist, or make a song for a big commercial or company.

Hope thats a helpful insight, if you ever have audio production or songwriting questions I'm happy to help! New on here, still haven't figured out how to post on dtube but excited that the music community is so active, really amazing.
Mahalo,
Sebastian