sejuani is in my opinion a very tanky champion that you need to play very defensive early
you can literal fed you're own adc with you're hard slows and stun

pros:

- you're hp in combination with you're slows and stun can help you win tf
- you can jump in or jump out a tf
- ganks early game are easy to escape with you're q well timed

cons:

- early game no dmg until lvl 8, but stil weak
- you need to feed you're adc, because you wont carry a game

basic rule.
try not to farm aggressive, you will get a lot of dmg from you're enemy.
also try to stay in lane to get exp, and only kill a minion if you know the enemy can deal dmg to you

in lane:

- try not to fight 1vs1 before lvl 5 you'll get more dmg dealt than you can deal.

team fight:

- try to use you're ult before you jump in.
- stay always in the middle of the fight you don't need kills you're the tank!