Dear Muta, based on what I have been reading, you have two choices - one is to have enough funds yourself where you can prove the origin (like salary, savings accounts, etc.), and the other is to get a US Citizen to be a sponsor. The minimum amount of money is 125% of the posted poverty line (currently just under $19,000 for a family of two for the 48 contiguous United States - slightly higher for Alaska and Hawaii). You might think you have enough money, but you may be asked to have a US sponsor as well. Depending on the financial strength of your sponsor, they may actually be asked to purchase a "bond" (a promise) in a certain amount of money that will be held in case you, the immigrant, get into financial difficulty. There are no hard and fast rules, based on what I have read), as to when they want what, but I would assume it depends on your age, and your particular skills (are you likely to get a job in the US based on your skills?), and probably, though I hate to say it, from your home country.
If you have a lot of money (and it does not have to BE in a bank in the United States, but you'd have to have access to it), then you may not need a sponsor. If you are borderline, then you might need a sponsor. Again - there are not any hard and fast rules, but it may be up to whoever interviews you.
I'm nervous. I am 55 years old. So I don't have too many more years to work before I am retirement age. So I am probably going to need a US sponsor (thank goodness I have one!).