If you do have your own blog, then you just need to follow the instructions here.
If you don't have a blog or Tumblr or any other kind of website, pop over to the "Can you grant a wish?" page and leave three wishes in the comments. (There used to be a form but I couldn't keep up with the demand as I had to make those pages by hand, so I'm opening the comments up instead).
Remember: Feel free to wish for whatever you want, but if you wish for something that someone else could fulfill (even if it's impossible for you to find a way yourself) it's more likely to come true.