Do I need a credit card?

A teacher of mine told me that I should get a credit card. I am 18 entering College as a First-Time Freshman this summer. My teacher told me I should get a credit card now so I can build up a good credit score. She told me, "Look this is what you do. You apply for a credit card and lets say you're going to spend $25 at a restaurant. You have the money in your pocket but you have your credit card too. So,, instead of paying with cash pay with your credit card. Then when you get home, put that money in an envelope and write *Bill Money for food* and add $5.00 just in case so in total it will be $30. Then at the end of the month when you get your bill, put the money in your bank and pay the bill with the money you saved in the envelope." Is this true? I'm not very exposed to credit cards and what not and if this were true. I don't get tempted to spend money. And if it is a good idea, where should I get a credit card from? And lastly, If I don't use the credit card for a couple of months, do I get charged even though I don't owe anything?