Getting Started No Experience, Want to get up and running Options

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Have read a number of web sites and really am non the wiser.

Will keep it short as I know I hate reading long forum text.

Equipment : iMac G3 500 Mhz 512 RAM slot loading running OS X.2.3 & Powerbook G4 800Mhz 512Ram running OS X.2.3

Experience : Fair Keybord player ; Serious Amateur Jazz Trombonist.

Want to : Compose and Play back Big Band Jazz (20 Voices) in such a way that it sound something like the real thing, with the ability to print the individual insturment scores off for players to read.

Have Tried : Demo of Sibelius - sound output was pants ! Was told by Sibelius that I needed some means of improving on the standard Mac sounds"

Confused by : Sequencers, Samplers, Midi Interfaces etc.

Please can anyone point me in the right direction without using any jargon (or at least explain what the jargon means and what I should be trying to aim for.) I am looking for value for money and something that will expand as I improve.