Determining your chart's ruling planet
The ruling planet of your chart is most often determined by the planet that rules the sign on the cusp of your first house (your rising sign/ascendant). However, if you have a planet that is conjunct your ascendant, that planet might be considered the ruler of your chart. The tighter the conjunction, the more likely a contender it is for chart rulership.
Another potential for confusion are the old planetary rulers of the signs. These include:
Saturn & Uranus ruling Aquarius
Jupiter & Neptune ruling Pisces
Mars & Pluto ruling Scorpio
Before Uranus, Neptune, and Pluto were discovered, Saturn, Jupiter, and Mars were the primary rulers of these signs. Modern astrologers have mostly replaced those old rulerships with the new planetary discoveries, but there is a strong case for the old rulerships. So which planet would you consider as your chart ruler if you have one of these signs rising? Investigate both potential ruling planets and their strength in your chart. Does one of the planets make more aspects in your chart, for instance? One planet may stand out over the other one as the likely ruler of your chart. However, if you can't determine one's strength over the other, consider co-rulership. Both planets may be equally instrumental in ruling your chart.
The Chart Ruler's Significance
So what's so significant about your chart ruler? The ascendant is significant because, among other things, it can tell you how you approach life, how you view and interpret the world coming in at you. The chart ruler is important for reasons that are similar -- the information you can get by analyzing your chart ruler will flesh out your understanding of your ascendant, giving you more detail about how you approach the world and how you interact with it. The chart ruler's house and sign can tell you a lot about where you are likely to find a lot of significant activity in your life.
The planet ruling your chart represents a piece of you that is elevated in importance. That planet could be said to characterize you, your outlook, and how you appear to others. Someone with Capricorn rising might be characterized as a Saturnian sort of person -- serious, hard working, concerned with achieving tangible success in what they strive for. Someone with Pisces rising might be considered a Neptunian sort of person -- open and kind. They might, however, also be considered a Jupiterian type, which also used to rule Pisces -- generous and optimistic.
Looking at those examples, they might not seem to tell you anything additional. After all, just knowing someone is Capricorn rising can tell you that they have a serious and hard working aspect to their personality -- hence the reason Saturn rules Capricorn, because they fit well together. In the cases of the chart ruler being a planet conjunct the ascendant, though, it becomes a little more important to consider the chart ruler's influence. If someone is Capricorn rising with Venus on the ascendant, the person will seem much more Venusian than a regular Capricorn rising person might appear -- someone who has more of a graceful charm and appeal about them. It can also be important to consider the ruler when you have someone with a rising sign that seems discordant with what you know about them. What about the Aries rising person who is soft spoken or the Gemini rising person who doesn't seem to say much? Finding out more about the house and sign position of the chart ruler can fill out more of the story.
The ruler's house and sign
An Aries rising person who doesn't seem to be characterized by the typical Martial bold energy is a perfect example of the complexity the chart ruler analysis can add to the picture. If this person had Mars in Sagittarius, they may display the sort of bold energy that seems like Aries, because Sagittarius is a fellow fire sign. However, if this person has Mars perhaps in Taurus or Scorpio, they are not likely to be as demonstrative or bold. Their technique of displaying their boldness will depend on what motivates the sign that the chart ruler is in. Taurus is much more laid back than Aries, and doesn't tend to get its hair on fire in the same way. Scorpio is much more guarded than Aries so wouldn't be as revealing as pure Aries rising might be.
The chart ruler's house position can give you even more information. The ascendant, as I've said before, is how we appear to others, how we approach life and the world -- our front door. But it also has a great deal to do with how we act on the world. It's our style of initiating contact with the world. The ascendant is always in action, from our most briefest and inconsequential interactions to our most important ones. Mars can be said to be the planet of action, and that wouldn't be incorrect to say, but Mars is just as much about what motivates you to act and how you attack and defend. The ascendant is the membrane in constant contact with the world around you.
Looking at the house position of the chart ruler can tell you a great deal about where the person in question projects him or herself in the activity of self-formation, in terms of the ego-inaction. The solar house may be the bank vault where the energy of life is stored, but the house position of the ruler tells you where it is primarily spent. Alan Oken: Rulers of the Horoscope
Returning to the previous example of Capricorn rising with Saturn as the chart ruler, we can add complexity by learning that Saturn is in Cancer in the 6th house. Cancer can tell us the sorts of things that a Capricorn rising person would feel a natural sense of duty toward. So they would apply their Capricorn rising nature of hard work and achievement toward Cancerian things, such as feeling a natural duty to take care of one's family, perhaps even acting as the patriarch or matriarch toward their loved ones, even if they are the youngest in the bunch. They naturally attune their responsible nature with the understanding that we need to protect each other.
The 6th house can tell us more about the circumstances in which this person often finds themselves and the activities that they undertake to achieve their Capricorn and Saturn in Cancer style goals. This person would spend a lot of their energy immersed in the tasks of daily living. They would feel identified with the process of perfecting oneself in order to accomplish the tasks that they have set for themselves. So the Capricorn rising person who has Saturn in Cancer and wants to look after their loved ones would probably take that obligation very seriously and set out to learn the skills that they need to do so properly. If one were to take care of their elderly parents, for example, they might find themselves taking classes on the nature of such care or learning about issues such as retirement and health care far sooner than they might ever have considered learning for themselves. There is also a general urge for them to improve themselves (the 6th house corresponds with the attitude of service as well as self-improvement for the sake of that performing that service), hence the desire to work harder at the tasks they have set out to achieve. It is not enough to do it, but it must be done well and with sufficient knowledge.
This is a complex subject with a myriad of different possibilities. When analyzing the nature of the chart ruler, focus on the rising sign, and then bring in the chart ruler's house and sign, which will add to but not replace the story of the rising sign. Try it in your own chart!