It's a more casual way of saying 'suru/shimasu'. (It might be more common in different parts of the country, or more commonly used by males, too; the man from Nagoya that I speak to almost always uses 'yaru' instead of 'suru'. I'm not sure if he uses it because he's a man, because he's from Nagoya, or just because we're friendly to one another, though.) It's the same as in 'Yatta!'
Unless you are chatting with good friends, use 'shimasu'. You might be seen as impolite if you use 'yaru'.