I've composed quite a lot of short pieces of music over the past 6 months (1-5 minutes in length) and they were all for solo piano.
Now I want to try something a little more tricky. I want to write a concertino (short single movement concerto) but I'm not entirely sure where to start.
I want to write it for solo piano, accompanied by Flute, Oboe, Clarinet in A, Trumpet in Bb, Timpani, Harp, Two violins, a viola and a violoncello.
Does anyone have any advice where to start, or how to go about beginning it?

I have four bars composed for the piano part so far (Its in A minor) and I quite like them but I'm not sure what to do next. Do I write for all the instruments as I go along? Or do I write the whole piece for the main instrument and then add the other instruments in one by one?

Help please

I'd actually recommend starting with a sonatina or sonata for 1 (or maybe 2) solo instruments plus piano. This will give you valuable experience of composing for instruments other than solo piano before you start on anything as complex as the concertino you've suggested.

If you want to compose for Clarinet in A it is worth also providing a transposition for Bb clarinet. The Bb clarinet is a lot more common, and even among diploma standard players there are plenty who only have a Bb.

Do you know people who play all of the instruments you've proposed? If not, then you might want to think about changing the instruments so that you can access 'live' players to try things out for you. A synthesiser can give you some idea of how different instruments will sound together, but it isn't really an adequate replacement for hearing the timbre etc live and also won't give you any idea of how playable things are on the actual instrument in question. As an example, someone had recently composed something for our orchestra. The climax was a 3-bar sffz trill on the flutes between top Bb and top C (I'm talking about the notes on and above the 5th ledger line above the treble clef stave) which also had a diminuendo down to pp. These are notes which are difficult to control precisely on their own, and it was impossible to move at speed between the keys needed for the two notes because it basically meant moving all of your fingers in different directions and sliding between keys. It's a shame that the composer hadn't checked before getting the music professionally printed whether it was actaully going to be playable because there was a conspicuous silence where the climax should have been.

Thank you for the advice.

I have written sonatinas before (Only four short sonatinas for piano) and I have written one piano sonata so I'm familiar with the form.
I think I may start by writing sonatinas to familiarise myself with writing for other instruments.
I'm only 15 and haven't been composing long so I still have a lot of practice to do!

Also I'm only at grade four piano and grade 4 theory, but that isn't stopping me from writing grade 8 pieces

Thank you.