What documents do I need to get a tourist visa for my girlfriend to come to the US?

Your girlfriend will need to apply directly at a U.S. Consular post in her home country that has jurisdiction over where she is currently living. An Information Officer at that Consular Section can provide her with detailed instructions as to completion of the application process for a B-2 Tourist Visa to come to the United States. In order to be granted such a visa, she will have to satisfy the Consular authorities that she does not have "immigrant intent" and that she has sufficiently strong ties to her home country that the visit will be temporary. You will want to prepare an Invitation Letter ("To Whom It May Concern") that she can take to the Consular Section in which you identify yourself and your complete contact information and in which you state that you will be responsible for any incidental expenses during her visit that she may be unable to cover to insure that she does not become a "public charge" during her stay. You may wish to hire an experienced immigration attorney to direct your steps if you contemplate a more formal relationship with your girlfriend in the future.