Hello and welcome! Most everyone on here will tell you to buy nothing until you have given the forums a great deal of combing. When I started not long ago I ignored this advice. It wasn't until I attempted to sell my hastily hand regged domains that I realized why that advice was something I should have considered!
Here are some tips that helped me break the ice with my first sales as a newby:
Ask yourself what the domain will be used for. End user is king. Why do they want your domain? Envision yourself building the website for each domain you consider. Is that domain necessary to turn a huge profit or can a similar domain be purchased for cheaper while accomplishing the same goal?
Browse auctions on a daily basis. See what domains are actually selling. This is where things started to really click for me. Watch plenty of listings on Flippa and see what gains traction. Try to guess what the domains sell for!
I hope that even a small portion of this advice has been helpful. I am a newby too and still learning.
Best wishes, good luck, and happy domaining! -Artifact