The Lock & Key by Sarah Dessen is not available to read for free online. However, there are several places you can find copies of the book at a very low cost, the obvious places being eBay and Amazon.
How to read Lock & Key online
If you wish to read the book online then try Amazon or iTunes, who both offer an e-book version of the novel. E-books are hardly ever expensive and you can read them on your computer, tablet or kindle as many times as you like. On Amazon, the E-book is only £3.49!
Or you can get a brand new paperback for only £4.99.
I have also found the book on eBay for only 99p! Although this is a second hand edition they are all stated to be in good condition. Or if you want a new copy they are on there for only £2.59 with free delivery!
If you wish to have a physical version of the book then first try your local charity shop. New books get brought into charity shops on an almost daily basis, so if you are patient, chances are you could be picking up the book for no more than a couple of dollars.
In my opinion buying a book, so that you can say it is yours, leads to a much more enjoyable read.