Have talent. No matter how much you practice, if you don't have any natural talent it will all be for nothing. Celine Dion has strong powerful vocals and a masterful use of pitch. Having at least a basic understanding of vocal scales will help you a great deal in your quest for singing perfection.
Learn the lyrics. Be familiar with all of Celine's hits and memorize the lyrics so you can sing naturally without having to follow words. Practicing a "Celine Dion classics" album on karaoke will be a good tool.
Study the master. To sing like Celine, it will help if you are familiar with her style and showmanship. Watch her live in concert (see resources) or study music videos and performances on screen. Copy her style as much as possible in your own performances.
Take some lessons. Nothing can be more beneficial than receiving professional tuition. A music tutor will help you hone your talents and be able to give constructive advice and guidance. Look in your local phone book or online for listings.
Get the verdict. You'll never know if you achieved what you set out to do unless you get some opinions. Perform for family and friends and ask them for feedback and suggestions for improvement. If you are very confident in your ability, performing at local events and talent shows will allow you to show off your skills.