Hello, and welcome. What type of coffin are you looking for. If you're looking for a modern one, a lot can be done with plywood and different moldings from the home improvement stores. If you're looking to make an old fashioned one or a "pine box", then a nail gun and some used fence boards will do the trick. Here are some links.
Well I don't have all the instructions in there yet on how to build this thing but we were the winners of the coffin contest on the forum here in a contest we had last year. Here's a photo.