One out of every five people on earth can speak English to some level of competence, which is quite a lot of people. English is considered an easy language for many. If you speak Spanish, Italian, Portuguese, French or any Latin-based language, this will help you a great deal in the learning process. However, you need to be aware that English is not an easy language for everybody. If your native language is Chinese, Japanese or Korean, for example, you may have more difficulty. The trick is to not give up.
In the United States you will notice a lot of different accents, idioms and vocabulary in different parts of the country. Unfortunately some Americans are not used to different accents and may have some trouble understanding foreigners.
If you’ve been learning British English, you will immediately notice the differences between the English and American pronunciation (although you’re probably already used to the latter from movies and music). Note that American English also has both spelling and grammatical differences from British English, some of which were made as part of an attempt to rationalize the English spelling used in British English at the time.
If you are coming to the U.S. to do business you should prepare yourself for a high level of communication in English. There is a big difference between the English used for tourism and the one required for business. Don’t forget to become familiar with the specific business expressions in your field of expertise. There are many resources you can use to do that, such as the internet or specialized books. You can also take a course in English specifically for business.