There are two main types of cover when it comes to replacing your goods. New-for-old entitles you to brand new stuff (or cash to the same value) if your insurer agrees to replace your damaged or stolen goods.
Other policies will cover you for the value of the goods at the time of the loss – these are usually known as indemnity policies.
An indemnity policy may be cheaper, but you only get a minor payout if you need to claim. So new-for-old is the best way to go. When you calculate the cost of your contents, factor in the value of your items as if they're new.