In champion select you actually have quite a bit of options. Depending on your playing style you can either ban a support or an adc that you hate playing against. While you generally can ban champions from other lanes I do not recommend it since you are at your weakest during laning phase and once you reach mid/late game if you position well you will be outranking every other champion and peeling with your w no champion should be able to kill you. You also have the flexibility of changing to support in champ select, if the matchup is one that you know you will struggle in

During the laning phase there are two things that you want to focus on, while you don’t have a lot of pushing power compared to other adc’s like caitlyn you have a heal and hard cc in your kit. In an optimal game you will want to freeze the waves near your tower allowing you to farm and poke safely. For trades you always want to go for short trades, hitting one enemy with an auto and then a q. This will get you a stack of your passive and with a freeze near your tower will allow you to stack it nicely into mid game. Your first back should ideally be a b.f. sword, boots and a control ward. You’ll want to constantly check the map after level six to either shiel your allies or finish of enemies which are low. The dark harvest will help a lot with this. You also have to focus on getting mist which will drop from minions after you kill them, this is most important becuase it is how you scale

For senna mid and late game starts at around 60 stacks. This is ususally a bit after laning phase ends and when you have two items. If you build Rapid Fire Cannon second you will have a nice amount of range and should have no problem outranging the enemy champions. You’ll want to position at the edge of the fight staying away from enemy cc since you are immobile and squishy. Once you reach 120 stacks of mist you will be able to outrange every champion in the game and even turrets.