I use this website for calculating astrology, myself, due to some of the other planets involved and included:
astro.cafeastrology.com/cgi-bin/astro/natalA few other things I'll point out:
1) If daylight saving was in effect in your country during the time of your birth, ignore it and calculate your birth-time as if it didn't exist. It can interfere with the accuracy of your ascendant and anything else which relies on time.
2) It is not advised that you let astrology put you off dating a person of a certain sign. People can fall on the more positive variant of a sign, or the more negative variant of a sign (or a bit of both). One Leo can be lovely, warm and generous, and another Leo can be a highly arrogant moron. A bad experience with an individual from a sign is not an indication that they'll all be like that.
3) By the same token, people with incompatible Sun Signs may in fact be quite well suited for each other, due to the other planets clicking with each other. A fiery Aries and a sensitive Cancerian may have Mercury's allowing them to communicate well, Venus's which allow them to love similarly, and Mars's that click quite well in desire and in bed.

So don't forget check what a person has under the hood- planet wise.

Then there are Cusps. Being born close to another Sun Sign can mean that sign has influences over you. The Cusp date ranges being approximately:
Pisces-Aries March 19-24
Aries-Taurus April 19-24
Taurus-Gemini May 19-24
Gemini-Cancer June 19-24
Cancer-Leo July 19-25
Leo-Virgo August 19-25
Virgo-Libra September 19-24
Libra-Scorpio October 19-25
Scorpio-Sagittarius November 19-24
Sagittarius-Capricorn December 19-25
Capricorn-Aquarius January 17-22
Aquarius-Pisces February 16-22
Having aspects of another Sun Sign influencing you can alter your compatibilities and personalities from those of the vanilla Sun Sign. A Sagittarius close to Capricorn will not be as jovial, and a Capricorn close to Sagittarius will not be as serious. You should probably count yourself as one side or the other, though- I'm still certainly a Sagittarius even with the influences of Cap'.