I'm curious if anyone can give me any insight into how to "cement" an idea into music - without lyrics. With that said, how does one go about selecting a key in which to express a particular idea and is there any specific significance to each key/note/interval/chord like there is in art (i.e. blue represents bla bla bla, orange...). To sum it up, how does one go about turning an idea into music, and how does one decide that music is the best medium for its expression?

Interesting... a concert pianist friend of mine swears by the key of Db but to be honest, it sounds the same as C to me. I suppose if you have perfect pitch then you may well associate different moods to different keys but this has to be so personal that I don't see how the composer has a hope of using it to his advantage. As to how to choose your key? Well, that's easy if you're writing for voice or an instrument with a certain range or particular attributes in different registers but more arbitrary when writing for wider range instruments like piano. I suppose it's all down to how it falls under the fingers. I suppose by all this that I mean that choosing a key is a technical rather than an artistic decision most of the time. Someone will disagree with me (this is, of course, the beauty of music).
As to how to turn an idea into a piece of music? I'm sure there are as many ways of doing this as there are composers. For me, I like to start with a structure that I can piece ideas into a bit like a jigsaw puzzle. Very often I compose in the order: beginning - end - middle so that I can see where I'm aiming for. Others write in a more linear fashion starting at the beginning and finishing at the end and just going where the mood takes them in between. Actually, I do this sometimes too. It seems the older I get, the less I know. And I like it

How to make an idea into a song:
1. Have an idea
2. Go to your creative place and/or time (mine is the basement at about 11.00 at night)
3. Let it flow. Just let it flow out.

How to choose a key:
This is not so much a choice. It comes from the flow of the song; as the idea builds, the appropriate key will become apparent.

How do you associate moods? It all depends on the idea.

Lots swear by D minor is the saddest, and I suppose C Major would be the happiest.  Then the fun starts as to what mode to use, for that bit, you're on your own

hmm kicking up a old thread ?

I have some thoughts on this...  writers used to say that the human ear had a natural tuning with Bb ,  but I'm not sure that still holds true today..   I would say that the mood would be more from the modes and feeling the overal song has... you could take a  song in Db Major and give it a light happy feeling to it , you could also take a song in  Cmajor and  give it  less happy feeling to it ... for me  the pitch (key) that the song is in is not as telling as modes and progressions used with what feeling and concept.. even though I love songs that are in the key of b minor and Bb minor for some reasonm and hard rock songs in E and F# (maybe cause those keys fit my voice for some reason). 

I do like to try to move songs around into different keys just to hear what they sound like .