Hi C.,
I work in a lab and I can tell you that your test will be more accurate the longer you wait. While some tests can catch a positive pregnancy before you even miss a period, it will be more accurate if you wait about a week after missing your period. (Having said that, I took two or three tests with each of my kids, one before I even missed my period too. It's too hard to wait!)
Another thing that improves accuracy is waiting to use your first morning void of urine. It will be concentrated because you haven't been drinking anything overnight. The more concentrated, the more likely that if you are pregnant, the test will pick it up.
Also, make sure you follow your test kit's directions carefully. Especially the parts about reading the test results.
Good luck!