Lake Country makes good pads, some of the best, that's their thing. Meg's are OK to, much better than they used to be. Buy a few because you don't want to mix products on the same pad and mark the pads so you don't mix pads on another day. You may need one for your heavy duty polish, one for light polish, one for applying wax, etc... Depends on how many steps you do. And it's best to buy the kit with the velcro backing plate and pads because it's usually cheaper. As for color, every brand has different color coding, the packaging will tell you what it's for.
As for where to buy, if you want instant gratification, look for auto paint supply stores in the yellow pages, most carry Lake Country as many pro's use that or Meguiars because of the huge variety of products they have so they likely have the pads too. While you are there they will likely have Meguiars and 3M polishes, the most popular and been around the longest and they may have Malco which I personally have used the most.
Good luck and post some pics on your first detail...