I have a girlfriend (boyfriend) in the United States. I want to visit. Any problems?

Yes. If the immigration officer at the port of entry learns of a romantic tie between you and someone in the United States they are likely to ask a lot of questions and possibly bar your entry. That is because such a tie strongly suggests good reason to believe you will not likely leave the US at the end of your period of authorized stay. Be prepared to answer a lot of questions if the reason you are going to the US has anything to do with romance.