so much people told me, "if you want to progress, you got to start make notes about your opponent". my question is, what thing your think is more important to write about them.

In general, I note how the player made mistakes or made an unusual play or plays any way out of normal. You want something brief so that you immediately know what their issues are.

For example, if a player limps with a hand that is normally raised (like AA or AK) I make a note "lmpd AK in co". If the player later does it again, I now know he/she is probably an inexperienced player and they limp with big hands so "limps with big hands" goes in the notes. From then on I know to be suspicious of their limps. I then also color code their avatar as an inexperienced player so I can tell from a glance what kind of player they are.