I've posted too fast on a number of occasions... happens to the best of us (and the worst of us like me...
Creating sounds... what a huge, great topic. The best way to tackle creating a sound is to start small and build up. Get familiar with the building blocks -and it becomes easier and easier to achieve sounds you're looking for.
Start with a single osc - filter - adsr envelope. Fiddle with it - see what it does. Add a 2nd osc - mess with the tuning a bit- you'll notice if you slightly detune it (just smidge) you'll get an interesting chorus like effect. Add another osc to modulate the sound...(usually labeled LFO) - this is a low frequency oscillator that is used to modulate the oscillators - making the 'move' a bit. Keep it small - keep adding things step by step. Try changing the waveform of the oscillator - sine, sawtooth, square... see what happens to your sound as you flip through them.
Effects... wow - there's a whole 'nother huge topic.
APK's right - taking a sound and tossing a bit of reverb on it will go a long way for getting the sound you're probably looking for. I also love the technique he mentioned - making a copy of the same track and changing the copy slightly - detuning it - or offsetting it by a few milliseconds from the original - changing the panning - changing the filtering...
For the "evolving" part - everything comes together - I like to be able to directly control the filter, modulation, on the synth... and control the effects playing part (I like to have my effects as aux sends on my mixer coming back into channels - so I can easily apply effects to the original or the already effected sound - create feedback loops - etc... ) while I'm recording - I will constantly ride the knobs on the synths, effects, and mixer to make it all evolve. This is just my technique - there are many many other ways.