You can take a pregnancy test as early as 2 weeks after having sex, 3 weeks is better. Use your VERY FIRST morning urine, your wakeup urine, for the best results. Testing early, false negatives are common.
A false positive is VERY rare.
Chances depend upon where you are in your cycle. If it's near when you ovulate, between about days 11 and 16 of your cycle, the odds are about 1 chance in 5. If it's near when you expect your period, the odds are very low.
Hope it helps.