It depends where you are from. If you are from any country within the EU (European Union) or EEA (European Economic Area) you do not need a visa to study in the UK. All other students need a visa to study here. CELT is a registered sponsor licence holder with UK Border Agency (UKBA) and can provide you with the type of letter required to obtain the relevant visa.
Different countries have different visa requirements, so for up-to-date information please visit the UKBA website.