Just so you know, there's an official Club Nintendo thread over here you can post these types of questions in.
It's followed by a decent number of people, and KingBroly is especially knowledgeable on the subject.
To answer your questions though, you need 600 coins to be accumulated between July 1st - June 30th for the given year. This means Club Nintendo "reset" a little over a month ago for 2011-12. If you had 600 coins added after July 1st of this year, you're in platinum status, which will show on the left side of the screen when you log into your CN account.
You do not need to leave these 600 coins in there for the entire year. Once you've made platinum, it stays. In other words, if you add 600 in the year, go platinum, and then order some things that take your total coin count down to 200, you will remain in platinum. It doesn't check the amount at the actual amount of coins you have at the end of the year on June 30th - it's a running total that measures it for the full year. Once you hit the threshold of 600, you're good to go.
It's best to "save" surveys (game registration codes) for the next year once you've hit 600 for a year. You can register some games and do all the surveys and the Intend-to-Buy surveys to help, but it's best to try and hit the 600 threshhold and wait until it resets on July 1st the following year. I.e., if you can get exactly 600 and have stuff left over for the next year, you're making the most of the registration codes. Registering 1200 in one year means that you could have saved 600 and used them for the next year to get platinum two years in a row without problem.
When you hit platinum, you have the choice of getting the platinum award OR the gold award. You do not get both. The only way to get both is to either buy them from someone else (lots of people on here tend to not really care for the rewards for whatever reason), go to eBay, or have two accounts through Club Nintendo, and make sure at least one goes to platinum and the other to gold. If you put 900 coins in one account for the year, you'll be platinum, but you don't get both awards - it's only one award per account per year. There was hope at some point that going platinum would get you both awards, but that's not the case. The gold award has been a calendar at least two (three?) years running, while the platinum award has varied between a Mario hat, Mario Status, and now Mario Buttons.
So in short: get 600, go platinum, you can't get both awards on one account, and spend them as you want once you pass the 600 threshhold, and you'll be good to go. The year runs July 1st - June 30th, resetting on July 1st each year. Try to be conservative and only get 600 a year, saving the other surveys for following years to ensure you'll go platinum each year.
Feel free to respond here, or in the thread I linked to.