What does it mean if a planet rules a house in your natal chart?

Every sign has a planet that rules it because the planet's energy suits the sign well. Houses have signs and planets that are associated with them; eg Aries/Mars/1st house, Taurus/Venus, 2nd house etc.

In the case of the 1st house, the sign here is the rising sign; this was the sign that was rising in the eastern horizon at the exact moment of birth but the planet that rules this sign will also show lots more about how you aproach 1st house matters and the same for all the houses. The planet that rules the 1st house can be seen as the chart ruler because it's so important but it's not always the case.

Planets are the "what" (energy) that's best suited for the house it's in, in a chart no matter what sign it rules; the house is the "where" (area of life) and the sign on the house shows how, (the style), you aproach matters to do with that house. If you have Taurus on your 8th house it is similar to having Venus in the 8th house; you may be good with financial affairs, use sex to get who you want, marriage could bring financial gains etc but no matter where your Venus is, if it's well aspected by other planets you'd probably tend to handle matters here in the best possible taste and in the most charming of ways. Anything sleazy will be a definite no-no.

Edit: oh I see; you have to find the "two" houses that have their cusps, (where the houses start), on the signs of Taurus and Libra because Venus rules both of these signs.

Rulership: Saturn is the domicile ruler of Capricorn and Aquarius (Not Uranus). What ever houses have Capricorn and Uranus on the cusps are influenced by Saturn. How that influence plays out is determined by position, strength and/or weakness of Saturn in the chart.

Let's say Saturn is in Virgo in the second and rules Aquarius on the 7th. Saturn is not very strong in Virgo and since Saturn has to do with scarcity, it indicates a scarcity of wealth. Why? Well Saturn rules the 7th and therefore the scarcity is related to 7th house issues. So your spouse spends you broke or you make a bad deal with a business partner.Other factors in the chart can mitigate this.

The idea that having Saturn in the first is like having Capricorn as your ascendant is spurious. It is the result of the bogus idea that planet=sign=house.

The chart is comprised of two wheels. One is the wheel of signs. There are 12 signs of 30 degrees of arc each and they are placed in order going counter clockwise, however the signs appear to move clockwise over the horizon each sign spending about 2 hours (on average) on the eastern horizon. The Sun is at 15 degrees Pisces today. So when the Sun was rising on the Eastern Horizon, 15 degrees Pisces was on that Horizon that is the degree and Sign of the first house cusp. I can't explain house calculation here, but there are 12 houses forming a circle but, depending on the house system they are not necessarily 30 degrees each. They are

superimposed over the wheel of signs. Each dividing line or cusp of the houses falls somewhere within a sign. The degree of sign is on the house. You need to get a beginner's book with diagrams to make this simple.