Go to the nearest immigration where you apply for extention with passport, tm30 (get from hotel) and pay a fine of 500b a day and than apply for the extension.
passport, passport size photo, tm30, a pen to fill in forms and time. agent will just go and do the same thing with you and you pay extra few thousand baht, cos i met a agent a few months ago.
bring tm30, passport, a passport photo, get the passport photocopied at the shop on the same floor, get 3 forms at the counter. bring a pen along. go early in the morning to avoid getting stuck there the whole day.