I think you'd better sit down. Had better is used in informa | English by mistakes

I think you'd better sit down.

Had better is used in informal styles when you give someone strong
advice about what to do in a particular situation. The situation usually exists at the moment of speaking and so there is usually a sense of urgency in the advice: 'You'd better hurry or you'll miss the bus.' 'You'd better ring your parents - just in case they're worrying about you. '

To give advice on a general situation or to say that one course of action
is better than another one, use should, ought to or it would be better to: 'Parents should teach their children to be kind to animals.'
'Rather than complain and risk upsetting her, it would be better to say nothing.'