People do sometimes find interpreting work without qualifications, especially if they have a language which is unsual or in a lot of demand, so you could start by putting the words 'interpreter bulgarian (or whatever your language is) jobs uk' or similiar in a search engine and see if there is anything available right now.

However, if your language is one that is more widespread in the UK, or not much needed, you are likely to need qualifications. To get good work, you may need to think about a special course - a diploma or master's degree in interpreting gives you the best chance. There are lots of these, so you should search universities and colleges in your area or look at online options. The British Council is a good source of ideas and information.

For more detailed advice you should contact the Institute of Linguists, whose website offers help on everything from studying interpreting from finding work.