The First House is the eastern horizon of the chart, the point where the sky meets the earth at the moment of birth. It is the house of the self: your physical body, your appearance, your basic temperament, and the way you meet the world. It is the house of the rising sign, the ascendant, which traditional astrologers considered the most important point in the chart after the luminaries.
In traditional astrology, the First House is called the House of Life. It describes the quality of the life itself — its basic vitality, its general direction, its fundamental character. The sign on the cusp of the First House, and any planets placed in it, describe the "flavor" of the person: how they look, how they move, how they present themselves to others.
A planet in the First House is a planet that is always visible, always present, impossible to ignore. It colors everything the person does, because it describes the basic stance they take toward the world. Mars in the First House gives a direct, assertive, even aggressive approach to life. Venus in the First House gives charm, grace, and a natural attractiveness. Saturn in the First House gives reserve, seriousness, and a sense of weight or responsibility.
The First House is naturally associated with Aries and Mars: the sign of the self and the planet of assertion. When planets are placed in the First House, they take on something of this Arian quality, becoming more direct, more immediate, more focused on the self.