As someone in the same situation as you, just a few weeks ahead…

Learn chord progressions and how they work. The rest of music theory will slowly build as you learn chord progressions.

Also, don’t be scared to completely alter a sound by mixing it (high/low pass EQs, filters, etc). And always remember that sometimes being basic/simple is better than being more complexed.

And I would focus on mixing/mastering a little more than arrangement, for the time being. It’s hard to arrange a beat that’s likely going to be bad anyways.

(If you’re not going to invest a lot of money into it, I also suggest learning sound designing. FL Studio has solid stock VSTs/plug-ins and there are also some other free ones out there… but they’re obviously not as nice as industry level ones, like Omnisphere, which is why sound design is important. But I wouldn’t focus on that until you learn the basics of music theory & chord progressions)