I am a pretty straight-laced person and have only been drunk a few times in my life in the safety of my own home, so I don't speak from extensive personal experience. But if you want a description of what it's like, especially for somebody who doesn't drink a lot, it feels a lot like you are being put under for dental surgery, or you are super super tired after pulling an all-nighter - it's kind of this weird, disconnected, floaty feeling. You feel a little bit disconnected from reality, like the things you're doing aren't actually happening. For me, my vision also pinches inward if I have too much and my eyesight noticeably distorts, which is usually my sign to put the glass down and cut myself off.
However, the most notable thing that happens when I am inebriated is that I talk a lot faster and a lot more, which is really noticeable for me because I'm usually a pretty quiet, introverted person. In addition to that, my thinking is often a little impaired and I can tell I'm not in a rational thinking space.
So writing a drunk character in this context would probably entail:
- Showing the character making irrational decisions, or thinking irrational and disconnected thoughts. People lose their grip on logic and do stupid things when they're really drunk. They often try to justify their "decisions" with a really weird string of logic that makes no sense to a sober person, but makes perfect sense to a drunken person. A drunk person's thoughts are probably disconnected and sporadic, rather than being a continuous stream of conscious thought. This character is probably having very short, weird thoughts.
- Different speaking style. In my experience, a somewhat drunk person will talk faster, talk louder, and lose their social "filter," and say things they would not normally say. They tend to be way more social and outgoing than normal. However, a really drunk person will start to go in the opposite direction and talk slower and slur their words, or they might be so drunk they can barely talk at all. Whatever the case, speaking is usually the first thing to be changed or impaired when you drink.
- Motor or visual impairment. If you get really drunk, your fine motor skills are the first to go, and all other motor skills are next. You can't walk in a straight line, your kinesthetic skills are impaired, and you struggle with locating objects in space. Once I got a bit tipsy and could not grab my keys off the counter on the first try, and when my friend challenged me to walk a straight line across the kitchen, I stumbled and had to grab the counter, so she had to walk me home.
In brief, your drunk character is probably more outgoing, confused, disconnected in thought and socially daring. She is probably going to be clumsy too. If she drinks more heavily, she could also lose the plot and become completely disconnected from reality, although I really hope that's not the situation you are trying to write for a teenager!