I wanted to set adwords account for my new website. I wanted to know what is the cost of per click will i have to pay in adwords. Can i define that visitors should be from USA? Can i define that my link should appear only on google.com search page?
Cost per click will be based on competition for keywords, Quality Score. Higher the QS, Lower the Bid. Yes, you can select the location (Country, City) for your campaign.
Depending upon your country you can pay by credit card, net banking, check. There are 2 methods- prepay & postpay.
The answer to your second 2 questions is yes. You can target the USA and then in advanced targeting select "only people in your targeted area. Then under the network settings select only the search network and then only Google Search. As to the cost per click that is going to very quite a bit. That depends on your quality score, the competition, the location and the time of the search. For example, someone performing a search in New York at 6pm might be much more costly than someone searching in Alaska at 2am. You can set limits to your cost per click but until you actually run your ads you won't know for certain.
This is probably the best search video ever. Hal Varian (Chief Economist at Google) talks through how the auction works. If you understand this, then do a little work to understand how to improve your QS then you'll be using Google Adwords about as well as you can.