House 1 begins at the Ascendant — the point of the zodiac that was rising on the horizon at the moment of your birth. It is the house of the relationship to oneself: outward appearance, spontaneous behavior, the way of acting before even thinking.
A planet located there strongly colors the first impression you give, almost as much as the Sun. See also the Aries entry, the sign traditionally associated with this house.
Angular house (with Houses 4, 7, and 10): the four angular houses mark the four cardinal points of the chart (Ascendant, Imum Coeli, Descendant, Midheaven) and are judged the strongest in classical doctrine — a planet in an angular house acts with more force and visibility than the same planet elsewhere.
Natural sign and ruler: Aries, ruler Mars — house 1 shares the tone of drive and initiative of the first sign of the zodiac, whether or not it is actually occupied by Aries in your chart (that depends on the time and place of birth).
Planetary joy: Mercury finds its joy in house 1 (Hellenistic doctrine) — thought and communication are considered at ease there.