I know a person's birth date, gender, birth place to calculate his birth chart and I calculate it on astroseek, now how do I find any weaknesses in his astrological birth chart? I don't understand much about it.
There's an astrology section here that talks a lot about it and even on that site you mentioned, they explain houses and signs.
But if you don't know this person very well, it's hard to say that they really do have these weaknesses; many things in the chart can manifest in a different way.