You can register the scjp exam by two ways. Visit pearsonvue -> select Information Technology as Department and oracle as vendor ->Register web Account--> fill all the required fields on the registration form " If you have voucher, enter it. else directly pay the amount online " --> Schedule Exam --> Submit
Take the print of the copy generated by the system[or you can find it in your mail which you have provided during form submission]
If you find any difficulty registering online , visit any prometric centre with voucher before 2 or 3 days you plan for the exam.