I'd say go as soon as you like and go back once six months have passed from the time of your unprotected conquest. I think HIV can take up to a year to show up in a test after you've contracted the virus.
I think planned parenthood offers free or subsidized testing. usually, you can call your city health official or any local hospital and they'll point you in the right direction
I have a sticky link at the top of the Love and Sex page with STD info.... if you have any additional sites to add or need more help finding something, let me know.
some STD's can be detected within the first 24hrs, some take 6mnths to a year to be detected. Regular STD testing is best. And remember, alway go and pick up the results. Dont just get tested.
WEBmd.com shows some of that information and if there are and especially if you are a woman - most cities have Center for Women's health. Also if you are in college, many colleges have Center's for Health and Counseling...they offer very cheap STD testing.