Don't buy in short stacked.
If you're playing cash games your goal is to maximize the amount you can win with your good
hands. If you have a spot where you have AA vs KK you want to get the full stack, If you don't feel comfortable buying in for the full 100BB it's a sign the stakes your playing is too high.
One exception to this would be when you become a very skilled player you can buy in with a shorter stack (after you've done lots of studying for short stack play) This is because you will be able to put other players in difficult spots where they are unfamiliar with the proper plays vs a smaller stack. But until you have a very strong understanding of poker theory you're not going to understand how to make the most of this advantage and wield it against others