Astrological compatibility depends on many different factors. The positions of the Sun, Moon, Mars and Venus in each person's chart, and aspects between these planets, are crucial. In India, it is common for couples to have an astrologer check their horoscopes before they get married to see if they are suited to each other.
Unfortunately, many people cannot afford the services of an astrologer. An alternative way of calculating your birth chart is by using an astrological program. You can download a program from the shareware section of the Astrology-Plus database.
However, it is still possible to find a basic level of compatibility based on Sun Signs. Because the sun takes a year to move through all twelve signs, it is easy to find your Sun-Sign from your date of birth. The range of dates for each sign are usually given in newspaper horoscope columns. Since the exact dates change from year to year, the range of dates is not always the same in different publications. A person born between the 20th and 23rd of any month should check his or her Sun-sign buy looking in an ephemeris. (This is a book which lists the positions of the planets on different days, and is available from any astrological book shop.)
The basic idea between Sun-Sign compatibility is that there should be a good elemental balance between the two people. For example, Water signs tend to go well with other water signs, or with Earth signs. On the other hand, Air signs are better suited to Fire signs than to either Earth or Water. Everybody's chart contains planets in all four elements, but there is usually a preponderance of one or the other. Because the position of the Sun is such an important part of your astrological make-up, it is possible to use your Sun Sign to give a rough idea of how compatible you are likely to be with other people.