As a huge Science fiction fan since the age of seven, I've done rather a lot of reading in the field.
First off, do you have an ipod or other mp3 player? If so, a website called www.escapepod.org releases a science fiction short story every week, completely free. This is a good way to read current new authors of science fiction and see what you like, without ever having to pay anything. When you're a young adult, your pocket book can be rather thin; free stuff is always an advantage.
Continuing on the free ways to get good books theme, a website called the Baen Free Library (at www.baen.com/library) offers free ebooks of famous and excelent published Science Fiction. I especially recommend _Rats, Bats, and Vats_, as well as the stuff they've got by James H. Schmit. You can read these ebooks either on your computer, or any portable device you might have (cell phone, pocket pc, etc) that can read text. If you find authors you enjoy, you can usually purchase more ebooks by them from Baen's sister website, www.webscription.net for extremely reasonable prices. I've been a happy Baen reader for a long time, and much of what they publish is okay for young readers.
If you're looking for printed books, one author you might enjoy is Mercedes Lackey. She writes fantasy, similar in tone to Harry Potter. I especially recommend starting with the Queen's Own series, _Arrows of the Queen_, _Arrows Flight_, and _Arrows Fall_. These books are quite popular; try asking your local librarian about them.
If you're looking for something more science fiction in tone, check out _Jumper_ by Steven Gould. It's a book with a teenaged character that fully explores the aspects of teleportation, from unlimitted wealth to government work. This is a recent book, so it shouldn't be particularly hard to find.
I also recommend the "grand master" to you, Robert Heinlein, but with some strong reservations. First off, some of his books are now out of print and hard to find. Second off, you probably want to stick to his young adult books. Many of his adult books are extremely preachy, and spend a lot of time dealing with rather explicit sexual matters (everything from sex out of wedlock to incest and prostitution) that both you and your parent/guardian are likely to find uncomfortable. They also have strong libertarian leanings, that aren't always based on the world as we know it.
If you think you'd enjoy a book that is science fiction, but with a strong philosophical message, check out _Right of Passage_ by Alexei Panshin, if you can find it. Be aware, however, that I'm not sure if the book is in print anymore.
Last, but far from least, try Piers Anthony. He writes light, easy to read science fiction and fantasy. Nearly all of his books are fine for young adult readers (I define fine for young adult readers as stuff that doesn't make me uncomfortable; I still qualify as a young adult reader :-)), and his popularity means that most of his stuff is either still in print, or can be found in your community library. His most notable series, xanth, comprises over 30 books. He also wrote several other more serious serieses, as well as many stand alone books.
escape pod: http://www.escapepod.org
Baen Free Library: http://www.baen.com/library/
Arrows of the Queen: http://www.iblist.com/book1451.htm
Robert Heinlein: http://www.heinleinsociety.org
right of passage: http://www.iblist.com/book9875.htm
Piers Anthony: http://www.hipiers.com