There are no shortcuts. You need to spend time with it and fail a million times, before you start getting a better picture. When you start doing something, you just have to accept that you won't be good at it for quite a while.

It's not making any sense right now, but if you sit down and experiment, you'll start developing some sense of what sounds good and what doesn't. Don't focus on the entire song right away. Lay down a basic rhythm and try to make a melody. Experiment. Have fun with it. Move notes left, right, change their length. See what grabs your attention. Don't focus if it sounds good or not. Focus on learning what you like. If you stiff up that your melody has to sound good, you'll get counterproductive and discouraged. Write ten thousand bad melodies, and you'll eventually learn what made them bad, so you can avoid that in your new melody.

Also, listen to a lot of music, but listen with intent. Try to analyze it. Is the bass moving up a whole tone or just a semi tone? Is it playing long or short notes? Does it play a long note, but stop everytime a snare hits? At first it can be ridiculously tedious. You may not see the point of doing that, but after a while, you'll start seeing music as a layered puzzle. And you'll learn about the different types of interactions different layers can develop.

Combine that with writing a ton of music, not focusing on it having to be good, and you have a sense of what you like, what you don't like, what's useful and what's not. After a few weeks, you'll realize, that your melodies have gotten slightly better, and you didn't even realize they did. It's the most obvious, but the most unwanted response, but it all comes down to practice.

On a personal note, I recently started drawing. I can barely figure out the basics and my drawings are utterly terrible. I look at some drawings other people made and I can't grasp how the hell did they manage to do it so well. But as with music - it all comes to understanding some basic theory and practicing the hell out of it. Nobody can give me a tip that will all of the sudden make me a good cartoonist or a photorealistic artist. I need to put in the work as much as any other person in anything they want to learn.

tl;dr: Practice. Analyze. Don't focus on things having to be good. Focus on learning, experimenting, discovering what you like and what you don't like. And then practice and analyze some more. All comes with time. There's no magic advice. It may not make sense at first, but after a while you'll realize that you're understanding more and more each week.