The Ascendant is the sign that was rising in the eastern horizon when you were born. To calculate it, you need your birth time and your location on Earth. Each sign is rising for only about two hours, so having an accurate birth time is essential. In quadrant house systems, the Ascendant forms the cusp of the first house. In Whole Sign Houses, it is within the first house.
It is an interface between you and the world that comes into play when people meet you in person. (Your public reputation amongst people who don't know you is your Midheaven.)
The Ascendant is generally unconscious; we don't know we are "wearing a mask"—it's been there from birth, it's there 24/7 in every situation, and we can't take it off. We can become more consciously aware of it, but it's always going to be with us. For example, if you have a Scorpio rising, your perception of the world will be about threat, survival, and finding and revealing secrets. Becoming more aware of your built-in filter won't change it, but you'll be able to account for its biases.
Any planets conjuncting the Ascendant will heavily influence it. For example, with Neptune on the Ascendant, the world becomes a dreamy place, and people can perceive you as otherworldly. With Venus on the Ascendant, you will be perceived as graceful (regardless of gender) and be oriented to the world around what you find beautiful and valuable to you personally.
I recommend looking at both modern and classic rulers if your sign has both. Learn more about the planet itself. This will help flesh out your understanding of your Ascendant.
Next, look at your Chart Ruler’s house and sign in your chart, and any hard aspects to it (conjunctions, squares, and oppositions). These can cast a "flavor" on your Ascendant. For example, if you have a Sag rising, but your Jupiter is conjunct Pluto in the 8th, you'll come across with a lot more Plutonic flavor than Sag Rising usually does. If your Jupiter is in Virgo conjunct Saturn, you'd come across as more grounded and serious. You'd still have an optimistic and enthusiastic outlook on life, it would just include these undertones. We all contain multitudes, after all. 😁
Transpersonal energy can be difficult anywhere in your chart, but due to the largely unconscious nature of the Ascendant, these placements can be particularly difficult to manage.
Uranus conjunct the Ascendant. You can come across as aloof, detached, or just odd, as well as unpredictable. You may instinctively resist any type of conformity or tradition, and be dismissive of the ordinary human needs of yourself or others (especially emotional needs or needs for comfort). To counter this, try to develop your Moon (your emotional self), Venus, and work on developing emotional intelligence. Accept that whether you understand them or not, humans have these needs (and you do as well), so you might as well learn about them and develop some ability to navigate them.
Neptune conjunct the Ascendant. You can have difficulty filtering out the world, making you sensitive to other people’s energy and easily overwhelmed. It also creates blurred boundaries and confusion around your identity. You can end up adapting to your environment and not be sure who the "real" you is, and have a passive feeling toward reality, as if everything is "meant to be" and you don't have agency. You may also be projected onto a lot, as you are basically a walking green screen. To counter this, try to develop your Sun (your conscious identity), Mars, and work on boundaries. And if you notice someone is unable to stop projecting onto you, you may need to reduce involvement with them.
Pluto conjunct the Ascendant. You can come across as intense and magnetic, sort of like Scorpio rising taken up a few notches. But you can also end up in power struggles or embroiled in drama without understanding why. It can feel like the world is a threatening place, and you have to constantly be aware of danger. Try to become as conscious as possible of how power operates inside you, and be honest with yourself about what you want in each situation. Be as open and transparent as possible with other people, and resist the urge to hide your motivations to keep the upper hand. In other words, rest your safety in whether others can be honest with you in return, rather than if you can control the situation.