I recently had my puberty break and now have to choose a Marja, I want to do taqlid of Ayatollah Sistani, please guide how should I go about it.
Since its my first time choosing a marja, do I still have to investigate about whether Ayt Sistani is the most learnt or not
If yes, then how am I suppose to do it?
And do I have to compare him with all the other Marjas out there, and which points should I focus on reach to the conclusion of who is more learnt
Once this is done and I am ready to do his taqlid,
Do I have to inform him on him website/email or do I have to take like an oath of declaration?
Can I follow him even if I have no khums to pay?

You just make the intention to follow him. No need to do anything else. For Islamic laws refer to sistani.org