Does a Bulgarian national need a visa to travel to the United States considering Bulgaria joined the EU ?

probably, the US isn't in the EU

Yes, and the US has nothing to do with the EU.

go to www.travel.state.gov find your nation on the list and see what you must do to travel here. even EU members need visa or passports to travel.

Yes, you will need a visa. Regardless of Bulgaria's status in the European Union, Bulgaria is not one of the 27 countries currently participating in the US Visa Waiver Program.

See the following site for more details:

http://travel.state.gov/visa/temp/without/without_1990.html#countries

Yes, you will absolutely need a visa. Although most EU nations (with the exception of Greece and all newly admitted nations since 2004, except Slovenia, which is already a member of the program), you will need a visa to enter the U.S. The EU has been working to get all EU states added to the program, however the US has indicated they will allow 5 new members on a trial basis in the next few years. I highly doubt Bulgaria will be one of these states due to their economic state at the moment.