When you receive the gift card, you can activate by following the instructions that are included. Some will activate when you use it and some may need to be activated on the phone. This is up to the company issuing the card. You must also sign the back of the gift card when you receive it.
The gift card is just like using a credit card. When you get to the check out or at the gas pump, you simply use it as you would the regular credit card. Each time you use the card, the amount is deducted from the total amount that the card was purchased for. If you are buying something that will be more than what you have on the card, you can use the gift card first and a second method of payment for the remainder. The card will state on the front of it if it is only valid in the United States if you are receiving a Domestic Use Only card. Otherwise the card is valid anywhere. There are millions of places to use the Visa gift cards.