If you are newcomer to US, (e.g. a student or a worker) and intend to stay for an extended period of time then it will help you to get a credit card. To begin with you must apply for a social security number. Assuming you went through that process, you also need a bank account. Then you need to apply for a visa credit card through your bank. (This visa card has nothing to do with your passport/visa!). Generally a mastercard is given to established applicants. Visa credit cards are given to most applicants with no credit history. Students get around $2000 limit on the card. Workers will get less (for profit making reasons). If you dont get an unsecured credit card, you can apply for a 'secured' credit card. You deposit a fixed amount of money in a term-deposit and get a credit card with equivalent limit. The deposit won't be deducted! You must pay separately for your credit card expenses. After a year or two your creditworthiness will be higher and you can close your term deposit and retain your credit card as unsecured credit card.

You may also apply for an American express card but the classic Amex card is a charge-card. It means you pay the balance in full at the end of the month and cannot carry over a balance to subsequent months. Generally it is easier to get an Amex card than other credit cards. I would not recommend discovercard to anyone since their marketing campaign and actual service or credit limits are in opposite directions. Personally I do not like such deceptive businesses.

A credit file is opened as soon as you apply for a loan or a credit card. This file will be with credit-reporting services such as Experian or TransUnion. Each year you may obtain a free copy of your own credit file from them. Your credit score (also known as FICO) is based on the contents of this credit file. The credit scores range from some 400 to 850 depending on which credit bureau is looked up. Beginners start with a score of 550. Best scores are generally above 800 but that takes 7-8 years of perfect credit history and big level spending and payment. The average credit score in US is roughly at 675. The higher you go in credit score the harder it is to go higher. If you miss one payment your credit score can fall by as much as 50 to 100 points. If you miss 2 consecutively or 3 payments in a year it might fall to 550 or lower. Your interest rates also rocket up if you miss even a single payment.

If you apply for a second credit card then the credit issuer will look at your credit file. If you authorize credit verification then the score falls by 5 points for each lookup. If you did not authorize a verification then the score does not get affected. Each prompt payment redeems the credit score by 3 points early on but slower as your score goes up.

A credit score is important for future credit applications such as home mortgages. A good credit can reduce the interest rates as well as give you a greater credit limit for big plans. In any case, it is recommended that you do not accumulate any balance in your credit cards since it can get out of control without your realization. Even if you are prompt in payments if your carry over balance goes beyond 40% of your credit limits in each card your score will reduce. This is because people who overuse their cards are likely to default in payments.