I haven't resigned yet – do I have to give them my reference details?
It is very rare that your current employer is aware that you are looking for a new position, unless you are in a contract role that has no chance of extension.
Now, you obviously can't give them your current manager's details – your manager will be caught off guard and may refuse you a reference. Luckily, most employers do not require you to supply references until after you have completed several interviews, meaning you will have the opportunity to think about who is the most appropriate referee.
In this case, think about your colleagues and who is in a position that has enough authority and knowledge of your role and you. If you feel they would be happy to provide a reference and will be discreet, arrange a quick meeting with them and explain your position. It's quite common and most people are happy to help.
Do my previous employers have to give me a reference?
No, they are under no legal obligation to give you a reference. If you ask an employer to give you a reference and they refuse, DO NOT give them to your potential employer. A person listed as an official referee, they are expected to give a reference. Your potential employer may think that something went drastically wrong / you have made false statements when interviewed. It could cost you the job
Can someone give me a bad reference?
In theory – yes, there is nothing to stop them as long as the information is correct. However, many employers would rather refuse to give a reference than give a bad one, for fear of law suit.
How do I make sure my reference will be good?
Think of your referees as your partners in the recruitment process. Sit down with them or make a phone call and go through the position and discuss how what you have done and your skills will match up. Be sure to flag key successes that you would like mentioned. Don't tell them what to say, but by spending the time, they will feel as enthused about the job as you are and give a better, more tailored reference.