Rainmeter for the things in top right and top left. Of course, it can do a lot of other things as well. There's an insane amount of different gadgets, skins and shizz.
I used icon packs on the internet for custom icons. I could recommend
DeviantArt. Just remember, the images have to be .ico, otherwise it doesn't work. Usually, icon packs are in .png format, and if you don't know how to change them into .ico yourself, there's always
this site.
Another way to get custom icons is through Rainmeter. Although, they can be a bit more of a pain in the arse to deal with if you don't know coding.
And the simplest way to change the start button is with
this. You'll just need to find something to replace the start button with yourself. Again, you could use
DeviantArt for that.
Those are the biggest aspects of customizing your desktop, I think. Woop woop! \:>/