They save all their Pokémon. It isn’t intuitive how to get rid of them, and the game doesn’t really explain all the pros and cons to doing this. Players really should learn how to do this as later on, it becomes a bit task repetitive to need to spend a half hour getting rid of 20 to 30 Rattata, Pidgeys and Zubats. Sort them all alphabetically from the Pokémon page found by clicking on the Pokéball on the main screen. Then click on the lower CP Pokémon of a certain type and donate all but say the two highest CP of that type. You’ll benefit from the candy. You’ll be able to evolve some of them later. You’ll be better able to see what you have. You’ll be able to catch more without hitting the limits.
Then evolve them all when you have a lucky egg to maximize your experience potential.
They waste Pokéballs on low CP Pokémon. To a degree, Pokéballs are a finite resource. You can find out how many you have by clicking on the backpack from the Pokéball menu. You collect them at Pokéstops, where you might get zero to six depending on your luck. Beginners sometimes see a Pokémon and then shoot five or ten Pokéballs at it. It has 10 CP and they have ten of that one already. After about four misses, if the common Pokémon with less than 50 CP is still not caught, just run away by clicking that button. No harm. No foul. You can use the balls for something else later.
They don’t use razz berries. If you see a Pokémon you really want, from the place that you throw, select your Pokébackpack and then chose a razz berry. Throw it at the Pokémon first, and then throw your Pokéball. This saves you a lot of potential grief later.
Source : quora