The question of online gambling legality makes the process difficult for players from USA and other places where gambling is prohibited. You can use a credit card to add money to your gambling account, but majority of American companies servicing credit cards do not approve transaction if they find out that money will go to a gambling website. That leaves players two options.
They can open an offshore bank account that they can use to transfer funds to online gambling site. But this is not a quick process, and banks will take commission for their services in the form of percentages of your deposit amount.
Some online payment services can send money to online casino accounts. Those services work similarly to Paypal, but the difference is that they can be used for gambling transactions. The simplest way is to order oversea money transfer, but that's a long process, moreover it's available not at every online casino.
Not all of those offshore banks and money transfer services can be used without any risk. How do you know who you can trust? Find information about a bank or a service to understand how long the company has worked, the richer its history the better. Conversation with a real person when you call a customer support would be a good sign. You can look for reviews about fraudsters and bad services on the internet. As soon as you have chosen the service, try sending a small amount of money to see if everything is ok. After making sure that everything is fine, you can send bigger sums. Regardless of the chosen payment system, it would be a good idea to send money back to your account as soon as you have a chance.