first of all have a look at the official Oracle's certification page which will give you a really good idea about the certification path you have to go through.
then you can have a read of ScjdFaq (contains tips, resources,...) to pass this certification
finally you can read several stories in the ScjdWallOfFame about ranchers passing this certification (which will give you a bunch of tips, an indication about the time you'll need,...)
And of course there is also this forum where almost every possible question is already answered a dozen times. So use the search engine and you'll find everything you'll need to know.