First switch your mobile on. It will search for a network and, after a short time, will display one of the networks that are available locally. If this doesn't happen automatically, you'll need to search for one manually. This is an option called something like "phone settings" - you'll find full details in your mobile's user guide.

Choosing the local Vodafone preferred network whenever it's available offers you the best value when making and receiving calls, so it's always a good choice.