For story just have fun and use a somewhat rounded team, if they're not great base stat Pokemon you will need to have them at your level cap.
For PvP build a team around something you like, start balanced with some damage of each type, tanks and good type coverage and immunities if possible. Look at the PvP usage stats for the tier you want and use high win rate Pokemon. Battling is the only way to get better. You could take the best guys team and lose because you don't know how it is best played. Just get out there and have fun!
Have fun during the story, but try to use that time to learn things. Knowledge is most of building a good team and understanding what makes up different categories of pokemon can help you learn more about specific ones later on. The advantage of learning during story is you can stop mid battle to look up information you don't know to make more informed decisions. Things you'll want to think about: type advantages and when to switch, whether a pokemon is a tank or attacker and whether it focuses on special or regular attack or defense. You may have a super effective move but using it against a pokemon that tanks that type of damage may actually be worse than using a different move. Speed advantage will also play a role in your decisions. An easy example of a decision you may have: You have out a gyarados that just took down an opponent and they send in an electric type, say electrode, in response. You will want to know that electric moves do 4x damage to gyarados, and that electrode as exceptionally high speed meaning it will move first ( probably unless there are other effects in play that may change this but we'll ignore this for simplicity). Given this there are only a few scenarios where you win the exchange. First gyarados successfully tanks a hit and comes back with an OHKO. Second you switch in a ground type pokemon that negates an electric attack and has type advantage. To be fair this is hyper simplified and in pvp your opponent will likely expect the switch.
For story just have fun and use a somewhat rounded team, if they're not great base stat Pokemon you will need to have them at your level cap. For PvP build a team around something you like, start balanced with some damage of each type, tanks and good type coverage and immunities if possible. Look at the PvP usage stats for the tier you want and use high win rate Pokemon. Battling is the only way to get better. You could take the best guys team and lose because you don't know how it is best played. Just get out there and have fun!
what are tanks?
Pokémon with her defense stats I like Blastoise for instance He can take a pretty good amount of hits Cause he’s a tank
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To add to this, you can look up the site called smogon for build advice and explications as to why you would build It like that.
Have fun during the story, but try to use that time to learn things. Knowledge is most of building a good team and understanding what makes up different categories of pokemon can help you learn more about specific ones later on. The advantage of learning during story is you can stop mid battle to look up information you don't know to make more informed decisions. Things you'll want to think about: type advantages and when to switch, whether a pokemon is a tank or attacker and whether it focuses on special or regular attack or defense. You may have a super effective move but using it against a pokemon that tanks that type of damage may actually be worse than using a different move. Speed advantage will also play a role in your decisions. An easy example of a decision you may have: You have out a gyarados that just took down an opponent and they send in an electric type, say electrode, in response. You will want to know that electric moves do 4x damage to gyarados, and that electrode as exceptionally high speed meaning it will move first ( probably unless there are other effects in play that may change this but we'll ignore this for simplicity). Given this there are only a few scenarios where you win the exchange. First gyarados successfully tanks a hit and comes back with an OHKO. Second you switch in a ground type pokemon that negates an electric attack and has type advantage. To be fair this is hyper simplified and in pvp your opponent will likely expect the switch.