first write a letter/ e-mail in which you ask for the information - try to keep it general and do not ask too many specific questions. Thus you're more likely to get what you want.
Then, the department/authority you have contacted should contact you within 20 working days to tell you whether the information is available or not, its cost etc. Then there will be another 20-day deadline to send you the information.
If the cost of the information is too high (mostly because it takes time and staff to get it), the department/authority will probably not get it for you. If the department/ authority does not want to give you the information, they have to explain why.
Also, it is good to know that journalistic material such as sources/ data is protected and can't be requested under the Freedom of Information act.
Here are a few websites about F.O.I:
Whatdotheyknow.com is a website that allows you to see some information requested under the Freedom of Information Act. You can browse any authority/ department and the website will display all the requests made to it. You can also write your own request through the website. This website will probably be useful in the future.
The Information Commisionner's office website is quite interesting as it is updated regularly about the new laws etc. As it's said in their website, they "uphold information right in the public interest, promoting openness by public bodies and data privacy for individuals" .