The road to LabVIEW Certification starts with the Certified LabVIEW Associate Developer (CLAD) exam. According to the CLAD webpage, the purpose of this exam is to demonstrate that the test taker has "basic proficiency" using LabVIEW. Since I've been writing LabVIEW code as my full-time job for over seven years now, I figured my work experience sufficiently prepared me for the CLAD. I recommend taking the sample exams available at the CLAD webpage above to make sure you are familiar with what kinds of questions to expect on the exam.
For me, the actual CLAD ended up being very similar to the practice exams, and I finished it very quickly (I think I was done with the 40 questions in about 15 minutes). If you have been writing LabVIEW code for several years, I doubt you'll need to do much preparation for this one. If you think you'll need more prep, I recommend watching the CLAD webcast and carefully studying the preparation materials. Both the webcast and the prep materials are available at the CLAD webpage linked above.
Additionally, there have been several threads on both the NI Forums and the LAVA Forums over the years that discuss the CLAD. Some threads have more general discussions, while others are specific to questions on the sample exams. Searching these forums for the term "CLAD" should get you to most of these threads.