It would be more helpful if you watched the first Gundam SEED series before starting with Destiny, or some impatient people would, no offence, really think you're messing with them. Here's a episode-by-episode synopsis to avoid making yourself look like an idiot to them.
http://www.mahq.net/animation/gundam/seed/seedtoc.htm
Here's some useful background info on the Gundam SEED world, quite impressive for an official website.
http://www.gundamofficial.com/worlds/ce/index.html