Please could anybody advise me on how to indicate my interest in a postgraduate position. i don't know wether to call the person in charge or to phone him. And again what information should i communicate to him i.e how shold i proceed with the conversation?

The procedure can vary from place to place. Firstly, if the advert gives instructions, follow those. If not, email the supervisor stating your interest and how you intend to fund yourself. If you are applying for a studentship, make sure that studentship is open to you (ie is it only open to UK/EU nationals).

Your first contact should be by email or by letter rather than telephone.