1) Why are there so many contradictions in the hadees? I’ve noticed some hadees say something while the other hadees contradicts it, so how would we know which hadees is true?

2) How are we supposed to understand religion when everyone has different opinion? How should we know who’s opinion to follow?

1) Contradictions happen for many reasons.
A- Sometimes people lie and make up things in the name of prophet

B- Sometimes mistakes can be made which bring up a contradictory message.
C- Sometimes the understanding of a hadeeth is contradictory. One example the prophet saying who ever I am his mowla Ali is his mowla. Shia understand mowla as leader while Sunnis understand mowla as friend.

2) That’s why Allah gave you the power of intellect and reason. You have to comprehend, not just follow blindly.