Passive : Absolution

Senna's passive is the most important part of her kit.
Every time you hit an enemy twice within a small time frame, you gain a soul.
Souls also drop from dead minions, they drop more often from minions you DONT last hit.
Each soul dropped from minions also gives 8 gold.

Each soul gives 0.75 AD, and every 20 souls you gain 10% crit, and 20 range.
Unless you're ahead, most of the AD goes towards your non-existant base AD.

In addition to taking souls, proccing your passive does %hp max health physical damage, scaling with level. There is a cooldown, also scaling with level, for how often you can proc your passive on an individual enemy.

Other than souls, your passive makes your attacks have windup, do 20% more damage, and give you a percent of the movespeed of the person you procced it on.

Your AA's are not projectiles, and can go through Yasuo's windwall.

You should be aiming to proc your passive whenever it's up, but don't die for souls.
Q+AA for quick soul, AA+Q+AA for PTA proc, W+Q for quick soul and root.
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Q : Piercing Darkness

Does damage, AA reset, slow and heal.
Scales with AD, while heals scale with AD, AP and Lethality.
Cooldown is reduced by one second whenever you AA.
Can angle off minions to poke enemy or collect soul out of reach, healing your team, or proccing passive and doing damage.

Applies On-Hit's and lifesteal.
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W : Last Embrace

Root, takes one second after hitting to actually root the enemy, applies one of the two needed hits for passive. Roots nearby enemies in a circle, when last hitting with W, it applies root at time of death.

Can be used on low-hp minion for insta-root, good in teamfights for rooting multiple targets.
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E : Curse of the Black Mist

Camo & MS, applies to those in the circle around you, if they step out of circle, they appear as a wraith, before being revealed after 1.75 seconds.

Good as a movement tool, and for jungle ganks, as you can hide them in your mist.
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R : Dawning Shadow

Map-Wide ult, reveals those around beam, does damage in middle, gives sheild to allies.
The sheild scales with your passive stacks, and sets you up for proccing it on everyone hit. Useful in teamfights, or hitting someone low who is backing in vision.
Aim from behind your team, to sheild them.

Fleet is for short trades, generally ran with lethality.
PTA is for longer trades, ran more with on-hit, as your combat style is more sustained.
PoM is almost required, as stacking without Q is risky.
You can replace the secondary tree with pref. runes with fleet and PTA.

Dark Harvest allows for more damage, and as you're support you finish ghost poro quicker
Taste of blood is for staying alive, and the relentless hunter is personal pref., I find when my movespeed is higher I die less.

Quick overview of the builds!

Lethality allows for heals and damage to squish,

On-Hit allows you to damage tanks too, and have more DPS,

Tank Senna is almost a meme, but it's viable if the game is going to go on longer, with decent damage as well. Your team will flame you, but you can run it against assassin heavy comps.

Laning Phase

Most important section of the game for Senna. Most souls you will get will be from laning phase, even more so if you're running lethality, as you stack slower with less AS.

You should be aggresive in almost every lane, if you can a soul, take it.
It's typically not worth it to run to jungle camps when they're clearing for an extra 1-4 souls, there's more to be got from minions and your enemy laners, also you leave your ADC pretty vulnerable when you do it.

Rush boots if needed, try to stay in lane longer if you can, don't die, ward as you're weak to ganks and Q your ADC when there isn't a chance you'll be using it soon.
Your heals are pretty useless when not running lethality.

If you kill early, just back, as you don't have time for that first plate or drag.

Souls from laning phase are something talked about a bit, some people say 80's alright, others say 100, aim for as many as you can without dying or losing objectives.
If you end up with low souls from laning phase, the game is recoverable, but you'll need to depend on your team more through the midgame.
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Mid Game

For Senna, midgame is dependant on your lane phase, if you had a bad one, you are very weak, very squishy, and basically free gold. Stay with your team, try to be around people who are clearing waves, and show up at teamfights. Ward objectives and bushes where enemies might wanna sit. Teamfighting is the same as most ADC's.
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Late Game

Your range makes your late game very good. You can attack at a range larger than your ADC, while, assuming you have had an alright game, doing nearly as much damage as them.
Stay with your team as everything oneshots you, and AA whenever you can.

Ward objectives, drag someone along with you when warding.