Subscriber Identity Modules (SIM) cards can be registered to take advantage of useful account and billing features like lists of recent calls and data use, special calling plans, access to personal unblocking keys (PUK Codes) and other useful tools. One of the disadvantages of picking up a Vodafone SIM card from a dispenser at an airport or railway station is that, although you are getting better rates than roaming with your home SIM card, you're still getting an off-the-rack rate. Registering the SIM card will allow you to choose better deals, too.
Browse to the Vodafone website (see Resources) and select the country you purchased the SIM card in, for example the Netherlands or the United Kingdom. Many Vodafone countries will have an English translation if you need it. Take a look at any deals being offered---sometimes you will get free credits simply for registering.
Insert the SIM card into your phone and make at least one call using the Vodafone SIM card. For some deals in some countries, Vodafone requires you to have made your first call.
Click on the deal, for example on the Dutch website, click on the link "Get a five Euro credit for registering." Enter the phone number associated with the SIM card into the website page and press the red button which will be labelled "Get the code" or similar. On the UK site, look for "Top-up" and choose "Register for online top up now." Look for the word "Register" on any Vodafone country-specific site.
Monitor the phone for any text messages from Vodafone and follow the instructions.