ERROR: Cookies are blocked due to unexpected output

This error comes up when I try to log onto my wordpress site: ERROR: Cookies are blocked due to unexpected output

I have tried to google the way to fix it, but I am only getting more confused, what with different codes to use etc.

I know very very little about how all this works. Does anyone have a very simple step by step instruction on how to fix it.

I have spent a long time already trying to work it out, and I have gotten nowhere.

Thanks in advance to anyone who can help on this!