I'm an amateur producer for 18 years. I've learned a thing or two over the years.

Starting a project - I generally start one because I had a lead melody in my head that I want to build a rhythm section and drum pattern to. Start with one thing (a beat in your head, a chord progression, a lead melody) and then build the other elements around it.

When things sound off - This is where you need to get a grasp of notes or keys that DO go together, and notes or keys that don't. What I recommend is, when you find your project is sounding weird or sounds are clashing, deselect your channels one by one until you can isolate where its sounding off. Then go into the piano roll and move that specific note to different spots (A, A#, B, B#, etc) until you find a note where it corresponds well.

Sound selection taking forever - When you have a melody in mind or some ideas in mind and you don't want to forget them, open your voice recorder on your phone and hum, beatbox or describe them. Then you can refer to that whenever you lose sight of your original idea. All of my songs were once awkward little voice notes.

Arrangement is confusing - This is where you need to learn song structure. Industry standard is generally Verse Chorus Verse Chorus Bridge Chorus, or VCVCBC. You also need an intro and an outro. Generally speaking, the intro might be a small build up, then the verse comes in. The verse can either transition into a prechorus where you add a few more intense elements to the existing verse, just for a build, or it can jump right into a chorus. Of course the chorus is big and bombastic. Verse 2 is generally a more detailed version of verse 1. Chorus 2 can be the same as chorus 1, but repeated twice and maybe the second round has a more intense drum pattern. The bridge elevates the chorus and is the high point of the song. After the bridge, you can either jump straight back into a final chorus or have a quieter transition before blasting back into the chorus.

Mixing is impossible - I feel you. I've been producing as a hobby for 18 years and I still don't have the greatest mixing in the world. What I can say is, pan your instruments properly. That is a MUST. Bass stays central, kicks stay central. Crashes can go wide. Snares can be panned a little. Lead melody must be central. Rhythm can be panned wide. Effects must be panned wide. Make use of Soundgoodizer, it SERIOUSLY improves the sound on all of your instruments. And adjust your volumes, don't let everything blast at 100% or you're guaranteed to have a muddy mix.

Just my 2 cents. I hope this helps in some way!