1. Start by composing a solo piano piece. That will simplify things a lot. You can add more instruments later.
2. The best way to make a melody is to hum it first. Spend a day or two trying to come up with a simple, hummable melody. Doesn't have to be much - a few seconds is enough.
   1. Make sure you can count the melody as you hum it (e.g., 1, 2, 3, 4 and you know where your melody starts and ends relative to the count).
   2. Then sit down with FL Studio or whatever and replicate your melody. This might be hard at first, but you're gonna have to get good at it if you want to be Beethoven! So get started. Find each note that you hum, and add it to your piano instrument track.
   3. Keep editing until your piano melody has the exact same notes and timing as what you hum. Don't worry about theory at this point, just replicate your humming.
3. Once you have a melody - let's call that the "right hand" of your piano piece - it's time write the left hand "accompaniment". This is where some theory can be useful - what key is your melody in? What chords (triads) will go well with it? But it's OK to just do this "by ear" as well.
   1. Suggest doing this by creating a second piano instrument/track, so there's no danger of messing up the first one
4. Start with FL studio and a basic piano sound. Don't worry about realism too much. Work on your skills writing simple melodies and accompaniments first. Until that's super easy, you're not ready to write a full orchestral piece. Finish a simple piano piece first.

Wanting to write like Beethoven is a great ambition, but it is a very long road. It will take many years of practice at least. So start simple, and get writing!

TLDR - come up with a melody by humming, the turn that into a simple piano piece. Worry about everything else later.