You can get the most accurate result if you wait at least one week after missing your period. In addition, the result might be even more accurate if the test is done first in the morning as the urine is far more concentrated at this time. The urine-based home pregnancy tests offer great accuracy. This accuracy will depend on several factors such as how early the test is taken, if the instructions were followed closely, when ovulation occurred in the cycle as well as how early implantation takes place, and the sensitivity of the test.
It is always best to take a pregnancy test after a late period to avoid false positive or false negative results. If you have irregular cycles or you are not charting your cycles, it might best to take the test after passing the longest menstrual cycle. For example, it would be best to take the test 37 days or later if your cycles are in the range of 30 to 36 days.
You can buy home pregnancy tests in every pharmacy or online (Amazon link).