It takes 6-12 days for a fertilized egg to implant, and another 2-4 for the HCG to be high enough to show up even on a sensitive test (like First Response or Clear Blue digital). The very earliest I would think you can get a positive is 8 DPO, I haven't heard/read anyone getting one any earlier. I would wait until at least 14 DPO, it is SO hard to wait, but you're only risking upsetting yourself by seeing a negative. I had 9 months of negatives because I'd test and start my period. I know how tough it can be, try waiting until at least 12 DPO.
I'm horrible when it comes to dpo, but the soonest you can test is two weeks from the last time you had sex. I think this is 14 dpo? Some have gotten results a little sooner, but you need to allow a decent amount of time for the hCG to build up.