I guess you mean a letter of invitation? The first thing is to check whether you need a visa and what kind of documents you will need. To do this you can contact your embassy or go the UK Border Agency website. If you look at the left hand side you will find the tab for visas and immigration; click on there to get the information you require or use the contact section to get in touch with someone directly.
If you are visiting a family member in the UK you may be asked for a supporting letter or letter of invitation. If you put 'uk visa supporting letter' in a search engine you will find several examples and templates that you can use.
If you click here and scroll down you will find under 'your letter of invitation' a list of points that should be included in your letter.