Some tips on preparing and cooking octopus:
Firstly, you can use the old fashioned / traditional method of flinging it against a rock to tenderise it, or beating it lightly with a meat mallet (remove insides first!), or (if you've bought it fresh, rather than pre-frozen) you can just clean it up, preferably remove the insides first, then freeze for at least 48 hours up to a week (obviously it will keep in the freezer for much longer than this, until you wish to use it) to tenderise it.