This couldn't be much simpler, thanks to this excellent website: http://www.writetothem.com/.
Enter your postcode and it tells you everything you need to know. Select your councillor, MP, or MEP and use the form to compose your message. Click send, job done.
Be polite and make your point clearly. You can copy a template message but it often comes across better if you use your own words.
You can also use this website to see more detailed information about your MP - what they are interested in, how they have voted, whether they have signed up to Early Day Motions, even subscribe to their updates. A bit of Googling will also find an MP's personal web page, and their surgery dates. Nothing beats a face-to-face meeting to get your point across.