When you make an observation, it will likely suggest a “computer vision” (sort of an artificial intelligence / machine learning thing) recommendation. If you accept that as your ID, then there will be no agree option for you, as each identifier (including observer) can only have one ID per observation. If someone else IDs as something else, then an agree option will become available to you, but you should only click it if you do in fact agree with them. If you do, that new ID will replace your old ID, and having two IDs the same will make the observation Research Grade.
I would suggest starting off cautiously, only putting IDs or agreeing with other identifiers if you actually believe the ID to be correct. Each “wrong” ID takes two or three “correct” IDs to bring the community ID in line with what it actually is, so you can minimise the community workload by being conservative in your IDs If you like iNaturalist and feel you will be active for a long time, then you can start to get more “assertive” in your identifying, knowing that you will be around to be a part of any discussion and re-evaluation that might happen. If you find that there is a split in the opinions amongst the community, and you yourself aren’t sure which species is more likely, then you can withdraw your ID and leave it to be debated by more knowledgeable identifiers.
My biggest advice would be to initiate conversation and ask questions at every opportunity… there is a fantastic wealth of knowledge, expertise and friendliness to be enjoyed here!