First of all, thank you for considering donating blood. The blood supply in the United States is entirely volunteer, meaning that it is only through the well-meaning actions of persons like yourself that a sufficient supply of blood for all patients in need can be guaranteed!
The short answer to your question is simply that you do not need to wait until the blood test comes back before going ahead and donating blood. When you donate a unit of blood, the American Red Cross automatically performs a whole bunch of tests on the blood to make sure that the unit is safe and acceptable for use. Therefore, as long as you are feeling well, you should be able to go ahead and donate.
There are a few exceptions when you should not donate blood. For example, if you are feeling sick you should wait until you are feeling well prior to donating. Similarly, there are a few countries that, if you have traveled to recently, you will need to wait to donate (because of the risk of malaria and other things). The staff at the blood donor center will go through these exceptions with you in more detail.
For more specific information about donating blood, and how it affects you, please discuss with your primary care doctor