I 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?

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You don’t need to do anything. You just follow his rulings and if there is an issue that comes up you refer to his opinions. You can find his opinions on the website Sistani.org

If several experts tell you sayed Sistani is the most knowledgeable, you can take their word. Today sayed Sistani is the most followed marji, therefore you can follow the suggestion of the experts and follow him. If you choose to do your own research you can.