I love John Green too! If you didn't know, he also co-wrote a novel with two other authors, called Let it Snow. It's not quite as good as his other stuff, but I enjoyed it
Here's some similar stuff. Nothing can quite match John Green, of course, but hopefully you'll like these:
Nick and Norah's Infinite Playlist by Rachel Cohn and David Leviathan
I am the Messenger by Markus Zusak
King Dork by Frank Portman
The Disreputable History of Frankie Landau-Banks by E. Lockhart
Audrey, Wait! by Robin Benway
The Perks of Being a Wallflower by Stephen Chbosky
Becoming Chloe by Catherine Ryan Hyde
Just in Case by Meg Rosoff
Spanking Shakespeare by Jake Wizner
I've read every one of these myself and I loved them all. Most of these authors have written other books as well, but I listed their novels that I thought were the nearest to John Green's stuff. If you find you really like one of the books, you can check out the author's other stuff, too.
I hope this helps!