Life as a Pokémon Trainer isn't easy, and there's a lot to consider when it comes to getting the most out of your Pokémon friends. One of the important things to consider is the nature of your Pokémon and how it will affect its overall development.
As they will stay with you longer and often form a vital part of people's teams throughout the game, the nature of starters like Chimchar, Turtwig and Pilpup is very important. While it really all comes down to personal preference, the way nature affects a Pokémon means that there are some nice combinations of Pokémon and nature that players can take advantage of.
A quick note, for all of these Pokemon, you want to avoid natures that will affect your speed, as attacking early is very important in the game.
Turtwig is interesting, as many natures work well with it. You really want to avoid a Jolly Turtwig and you will find yourself in a good position. You can definitely be happy if you get an Adamant Turtwig due to the extra attack stat.
For Pilpup, a Modest nature will result in a higher SP. Attack, which is great. However, he sacrifices a normal attack for this. This is fine, as normal attack moves won't really be part of a solid Pilpup's move arsenal.
An Adamant Chimchar will trade some Sp. Attack for standard attack, so if you're going to be using a lot of standard attack moves, this is the way to go.
A Jolly Chimchar is going to trade some Sp. Attack for Speed, which means they will make more attacks more often by nature than the first attacks.