Picking the right nature is extremely important, especially for PVP. In order to pick the best nature for your Pokemon you first have to look at what your Pokemon uses the most and what can be spared; You can do this by first looking at the base stats of your Pokemon (you can find these on the Dex page here or in game). 

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If you have decided you want to have an offensive nature you can look at the moves your Pokemon uses (as the game is in its Alpha stage the Pokemon's move pool is incomplete) looking at what moves your Pokemon uses can quickly determine the nature you want if the moves mostly require contact (Physical Attacks) you will want to go for a +Attack nature however if they dont require contact (Special Attacks) you will want to go for a +SP.Attack nature.