This haggling thing has got me worried, I've never done that before

OK this is a silly question and should likely start a new thread.
But how do you know what is reasonable, or if they say £10 do I give a silly low offer and what

Do they get upset if you offer a low amount

Well if it gets too stressful I could always give up shopping.
I know this sounds silly but it really worries me

Any advice would be much appriciated