Hmm...I love octopus!
Actually here in Italy it's very popular.
The easiest way to cook it is by boiling. Clean the octopus, beat it with a meat beater (hope this makes sense in English-it's what we call "batticarne") to soften fibers, then make a court-bouillon with vegetables and your favourite herbs, add a cork if you like (we had a thread some time ago about the utility of a cork to soften octopus meat!), put the octopus in and boil until tender, 1 hour or more according to the size.
There are two schools about boiling octopus:
1)It has to be put in hot water.
2)It has to be put in cold water.
I'm for the 1), but it's likely it makes very little difference.
Cool it down into the broth, cut the meat in cubes or slices and season with chopped garlic and parsley, lemon juice and EVOO, S&P. Mix with boiled potatoes if you like.
Another very tasty option is OCTOPUS STEW:
Fry very gently in EVOO some chopped garlic and few salted anchovies until they're dissolved. Add a handful of soaked chopped mushrooms and a handful ground hazelnuts (or whole pine nuts) and fry other 2-3 mins. Dice in cubes your octopus and sautè it with the sauce about 5 mins, deglaze with white wine, then add a VERY SMALL amount of concentrated tomato puree (never more than a couple of tbsp!), a glass of water or fish stock, cover and stew gently until tender, adding more water if required. It will take about 1 hour. Five mins after turning off the heat, add a handful of chopped parsley. Adjust salt if required and enjoy!