Hey there!
Well, I'm also fourteen, and I actually read 'The Lovely Bones' when I was twelve. I did enjoy it and I do recommend it because I think the plot is pretty original and it's a great read. The movie was really good too, although a bit different.
I guess it just depends on your maturity level (because there is some mature content in it) and what kind of books that you're into.
I haven't read 'Dear John' myself, but Nicholas Sparks is a very talented author (I loved 'the Notebook'), so it's on my to-read list.
I say that you should read them both anyway, because you'll never know unless you try. =]
These are just some of my favourite books which I definitely recommend. I'll attempt to write a short blurb about each of the novels (but they won't do the books justice):
1) 'The Time Traveler's Wife' - Audrey Niffenegger.
The Time Traveler's Wife is about how a man, called Henry, and his wife, Clare, try to cope and live normal lives although Henry has a genetic disorder; he unpredictably and involuntarily time travels which often leads to risky events.
2) 'Noughts and Crosses' - Malorie Blackman.
Callum and Sephy are best friends. But Callum is a nought (a second-class citizen), and Sephy is a superior Cross, and Noughts and Crosses simply don't mix, therefore their relationship is frowned upon by society.
3) 'Pride and Prejudice' - Jane Austen
The classic tale about the middle-class Elizabeth Bennet and high-class Mr Darcy. When Elizabeth first meets him, she thinks that he is arrogant and rude, after he snubs her (without realising that she was in hearing range). Mr Darcy, however, finds himself increasingly attracted to her.
4) 'The Book Thief' - Markus Zusak
Narrated by death itself, this book is very original. It is about a girl, Liesel, living in Nazi Germany. She finds a book in the snow near where her brother was buried and from then onwards, starts to fall in love with books and begins to understand the power of words.
5) 'Memoirs of a Geisha' - Arthur Golden.
You'll find yourself in 20th century Japan while reading this extraordinary story about geishas, and the life of one geisha in particular, an orphan, Sayuri.
6)'My Sister's Keeper' - Jodi Picoult.
Anna Fitzgerald was genetically engineered to be the perfect genetic match to her older sister, Kate, who was diagnosed with leukemia, at the age of two. Certain parts of Anna's body have been donated to Kate, but when she is thirteen, and Kate needs a new kidney in order to survive, Anna decides to sue her parents for making her donate her body parts to Kate against her own will.
They are all remarkable novels, and are very well-written. However, the first, second and fifth do have some adult scenes, so be warned. ('The Time Traveler's Wife' especially)
Hope this helps!
Happy reading. =]