I’m Jinyoung, the Teleport scout for the Bay Area, but can help you with visa questions.
However, this largely depends on your personal circumstances. The easiest way is to enter the U.S. is with travel visa (or get waiver. If you are citizen of Spain, then you are eligible for visa-free 3-month stay in the U.S. More information : http://madrid.usembassy.gov/visas/travel/without/waiver.html)
There is no special visa for foreign freelancers - they often stay here on travel visa or open up their own business to receive Business Visa.
I need more information from you to provide the other possible options for you. I understand that answers to these questions might be too personal to post it here, so can you be in touch with me via Scout System?
Here is the link to Teleport Scout : https://teleport.org/scouts/
1) How long do you want to stay in the U.S.?
2) Is your boyfriend U.S. Citizen/Green Card holder? If not, what visa does he have?
3) What is your job?
4) Do you plan to work for U.S. employer at some point?
5) Do you need other help besides visa for your move to the U.S.?
Moving to the new country is a lot of work and can be stressful. But we are here to help you :)