Plasuma!!!

There are no rules, certainly, but there is a general syntax that can determine whether or not a melody will be any good to our ears. Music is a language like any other - it evolves as new vocabulary are introduced as well as through cultural change, but the core grammar remains expected and mostly consistent. So it would help to learn the grammar of this language, even if you want to speak a different dialect with a totally unique vocabulary.

Melodic composition is a craft. It is one thing to write down a ditty you have swimming in your head, it is another matter entirely to determine how you should develop it into a complete piece. Some melodies will be versatile and effective, others will be unworkable and sound contrived. Inspiration is unreliable, and this is why the the craft becomes necessary. Study pieces from composers such as John Williams and Richard Wagner - not to listen for the "inspirational" melodies they've written, but for what they do with those melodies. While learning from their works, you should seek to understand why their melodies are built the way they are and how they are characterized for specific purposes. They were not invented willy-nilly on the spot, they were crafted carefully to fulfill a specific goal. There is a syntax that can be learned in studying this way, but you do not necessarily need to copy their methods of melodic embellishment. To add: the best authors are those who have read enough books to understand how a sentence can be written.

CheeseLord put it quite well. However, be mindful that jumping into the subject of music theory will not get you writing epic music immediately. So you may be initially disappointed by what the knowledge will inspire - but it is essential information to internalize. After you have a familiarity with music theory (namely intervals, chord theory, reading / writing music, and the standard nomenclature), start working your way into the subjects of counterpoint and harmony theory. In my opinion, counterpoint should come before harmony, but also studied in parallel. During this time (yes, right now), you should be creating music in any way you can manage to do so (with a digital audio workstation, on staff paper, with a voice recorder, or just in your head as an idea). Even if you do not think you understand it, just do as best you can and don't let the results discourage you. You will learn best through study, practice, and experimentation - but only if you are persistent.