You will need a proposer and a seconder. If you want to be a Committee member, you would normally ask the President to put you forward. I would suggest doing this even if you are consciously standing as a counterbalance to the views of the President or other team members. It is good to have constructive disagreement on a Committee, but you need to be prepared to work as a team throughout it.
If the President doesn’t put you forward (or you don’t want to ask them to), anyone else can nominate and second you.
Be sure to approach your proposer well in advance, because they may need to make a speech proposing you – if the election is contested – and you will want them to be prepared. You can help by providing them with a few bullet points about you and why you’d be good for the role – it will be appreciated!