My suggestion (and also covered by TONE) would be to pick one of your favourite songs and just analyse it and find out what makes it tick
I am not talking about identifying the harmony, chords, scales etc. - just listen with a pen a paper and make notes/observations on the following:
Most "songs" are written to a defined structure (eg verse, chorus, verse, chorus, bridge, chorus)
Even the most complex orchestral/film score type music can usually be broken down in to sections (eg A section, B section)
The key to making most music engage and become memorable with listeners is repetition
The key to making most music engage and become memorable with listeners is repetition (
)
So when you analyse your favourite pieces I think you will be able to identify repeating sections or motifs
When you listen to your piece notice how (and when) the main motif (melodic hook) is introduced, notice how it is usually built upon in each following section, note how it is subtly varied to maintain interest, note how the music completely changes feel before returning to the main motif
This is a very big generalisation without listening to your reference tracks but I think it is a safe bet that a similar structure has been followed (it is used in pop music, classical, orchestral, film scores, jingles etc. etc.!)
I would therefore suggest that you start at the very beginning and just focus on developing a simple chords sequence and a catchy melodic motif (hook)
Although focused on "proper songs" this podcast and the notes in the forum post might help you consolidate your theory knowledge before embarking on your journey
The theory (and techniques) translate to instrumental music
http://www.songwriterforum.co.uk/index.php?topic=9447.0 Once you have your "foundation" (a simple chord sequence and a memorable, interesting and catchy melody) you can then learn the different techniques to embellish and expand this core idea into a full score - but IMHO you need this in place first so you have something to build your "orchestration" around
The best pieces have an orchestration that SUPPORTS and reinforces the core concept - eg if you come up with a happy, major key, jaunty melody it will guide you as to how to build the orchestration (faster tempo, pizzicato strings, light percussion, bassoons [they just sound "funny"!], tubas, bells, chimes, plinky plonky piano)
If you come up with a "menacing" minor key melody then you may want to look at a slower tempo, cellos, hard hit lower register piano, big crashy drums, legato strings, that screechy/slide string sounds, timpani, etc. etc.
IMHO the main melody is ALWAYS the most important part of any song so starting with this and buildimg from it will always be a good place to start as everything that follows can then support this key part