This amazing guide is from Imgur user: LMG89
General guide to PoGo
Step 1: Look at the CP curve of a pokemon. The CP curve (sometimes called arch) determines how much potential a pokemon has to increase it's CP by powering up and tells you how far the pokemon is in increasing that max potential. Your trainer level determines what the max CP is and how far up the curve you can power up a pokemon (if you've reached the limit, it will say 'Trainer level to low to power up' or something similar when you try to power up). Your trainer level also determines how much CP a wild pokemon has and how far up the CP curve it is. This doesn't mean you won't see low curve/CP pokemon at higher levels, but it means that it's more likely for you to see a high curve/CP pokemon.
Step 2: Determine which pokemon you want to level up. Ok, quick question. Say you have 50 Pidgey candy and let's pretend the Pidgeot on the left is a CP240 Pidgeotto with the exact same curve. Would you evolve the Pidgeotto or the Pidgey?
If you've chosen to evolve the Pidgeotto, you chose poorly.
If you've chosen the Pidgey you're a quick learner.
One might be inclined to use his candy for the pokemon with a higher CP, but it's the curve that's most important. At least across evolution stages it is. In this example this Pidgey would have evolved into a Pidgeotto of around CP280 and later on it would become a Pidgeot of around CP650-700. The Pidgeotto on the other hand will evolve in a Pidgeot of CP510.
Keep in mind though this is on a Iv114 trainer. The reason why I'm stating this is because a trainers Iv' affects what the maximum CP is and what potential a pokemon has. I can have two Pidgeys at the exact same spot on the curve, one caught at Ivl 10 and one at Iv114 and the level 14 has a higher CP (I'm still not certain if this is because of the trainer Ivl at the moment you caught it, so take this with a grain of salt). Other factors are the pokemon's weight, height and HP. Sometimes you've got to make hard choices, pick the pokemon with higher HP but slightly less CP or pick the pokemon with higher CP but slightly lower HP.
And sometimes, something strange happens. For example you have two Pidgeys, one with CP207 and one with CP204, but the CP204 is further on his curve than the one with CP207.
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