Astronomically speaking, your first house is that one-twelfth sector of the local sky just below the eastern horizon occupied by planets two hours before they rise. Astrologically it symbolizes early childhood conditioning, and describes self-awareness and the experiences most necessary for its development. The first house is the mask you wear in relating to and projecting yourself into the world. It rules physical appearance, body type, conscious self-image, and how others see and experience you. It’s the window through which you view the world, and through which others receive their first impressions of you.
Whereas your first house is the window through which you view the world and through which others receive their first impressions of you, the sign on the cusp of your first house (your Rising sign) is the particular tint on the glass of that window, coloring and modifying all energy that passes into and out of your being. In fact, all of your personality functions (planetary energies) must eventually emerge through and be colored by your first house cusp if they are to emerge at all. Planets in the first house are parts of your self-expression. The more planets here (except Saturn and Neptune), the more you are involved with putting out your own energy rather than experiencing that of others, hence the greater your subjectivity. Thus the sign on your first house cusp conditions the way you look and how you express yourself.
You're very receptive to your environment and a psychic sponge. You're also very permeable to negative energy -- and can find yourself compulsively drawn to it -- so be aware of soaking up negative energy from other people. You can go a number of ways with such a strong position of Neptune: drugs, theater, music, dance, religion, mysticism are some. The theme is piercing the veil of the material world and going beyond appearances into universal truth, becoming at-one with the way things are. Your life can be disoriented and confused until you give up trying to satisfy yourself and gain some spiritual orientation. Learning to surrender your ego through such disciplines as meditation, Tai-Chi, yoga, psycho-drama, or emotionally based psychotherapies would be centering and helpful. Learning to not react to life emotionally would also help. You're being asked to sacrifice the personal and what your separate self wants. You’re also a real charmer, and able to enchant others and bring them under your spell. You're idealistic, have a strong imagination, and are able to see and feel the unity behind peoples and things.
If Neptune is within 10° of the ascendant it is conjunct the ascendant, and much of the foregoing is even more strongly emphasized. If Neptune is in the twelfth house AND within 4° or 5° of the ascendant, then it is said to be on the "dark side" of the first house, and it influence will be felt in the first as well as in the twelfth house. The closer a twelfth house Neptune is to the ascendant, the more it will be interpreted as though it were in the first house; when less than 3° from the ascendant, a twelfth house Neptune expresses itself primarily -- and eventually entirely -- in the first house.