The first impression for someone is their Rising Sign .. whatever Sign was rising over the eastern horizon at the MINUTE they were born, as seen from the latitude of their birthplace.
Since every one of the 12 zodiac signs will rise in the east, over any 24-hour period, unless you HAVE an exact birthtime and do up that person's chart, you cannot find their Rising Sign.
But the Rising Sign is ONLY the mask we wear around people we don't know well or are not relaxed around. So as you get to know the person better, and as they start to relax and be themselves around you, THAT is when you start to see the influence of ALL TEN of their planets .. .not just their Sun. (the "Sign" you find from the month-day of birth is only the Sign that Sun was moving through. It is easy to find, but it is NOT what astrology is about. Everyone has all 12 Signs in their birthchart, and 10 planets ... and no two people have identical birthcharts All Sun-sign astrology is good for is making money off a public that doesn't understand astrology.