When you are double sure you are not wasting your money. And even at that, buy only few and keep learn. If as you learn more you realize the first few names you registered were not good,l, you will take consolation in the fact that it was part of your learning.
Also as a beginner, try to always use the appraisal section whenever you register names. This will help you get the opinion of experienced domainers about what you're registering.
Good luck.