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If you are from the UK or a number of other countries, you can go to Argentina for up to 90 days without a visa. This also applies if you are going to work there for a short period, and you can usually extend your stay for a further 90 days when you are in the country - you can get more details here, you may have to translate the page).
However, if you know that you will be staying for more than 90 days you need to apply for a work visa before you go. For this you need a number of documents as well as a valid passport. If you click here and scroll down to the 'obtaining an entry permit and work visa' section you will see a full list of the paperwork you need.
You will then need to ask the person you are working for to apply for a visa for you. For any other help with your application, you should contact your nearest Argentinian consulate or the embassy in London.