Hey Tim! If this is your first attempt, it’s really quite nice! So, you invoked the Toccata and Fugue in D; were you just trying to sound like something classical or were you aiming for it to sound like Bach? I am by no means a master composer myself, but some points I noticed is that a Fugue is a specific style of imitative counterpoint writing, though without getting too into the theory, counterpoint is writing two or more melodies simultaneously and is something that is a complicated topic and requires a fair bit of serious study, but is essential to writing Classical/orchestral music.
Another technique to consider is modulation; this is the practice of changing keys during a piece. This is one way to keep the music interesting and can be used to start a new section of music or to add some variation to a melody from a previous section.
Also, the use of harmonic cadences. This is a specific sequence of chords to give the music a feel of incompleteness/continuation or a sense of completeness/ending. For example, you’ve no doubt heard the classic “da…DA…DAAAAAAAAAAN” ending in many Classical pieces. This is the IV-V-I cadence, or F-G-C chord progression in the key of C major.
Here are a couple of links that you can use for reference to some of these points. The first is Camille Saint-Saëns’ second symphony. After the intro when the faster part starts with the strings, is an example of Fugal counterpoint; each section plays a similar melody line at a different harmony to start while the others play a different melody underneath. The second is a solo piano piece of my own composition. You can hear both counterpoint lines and how I vary the melody by offsetting it by an octave, high, then low. There is also a slight modulation up toward the end of the B section and return to the original key.
I hope this can help you and good composing! I really liked your piece!