Try getting to know her more. This will be VERY hard if you are not any classes with her and will probably not work out too well unless one of your friends is friends with her. :(
I'm sorry to say this, but unless you know someone who knows her and are in the same classes as her, you might just have to deal with the fact that she won't be able to go out with you because of it. It's not because you're a kid, and you're not, you're a teenager, it's just the fact that it would be really hard to spend time with her because of your scheduales. Don't worry about it though; try looking for someone in your grade or a sophomore that is relatively the same age and has the same interests. In a few days/weeks, you'll notice someone else that stands out in your life more than her, and someone who you are probably more compatible with too, so that's better than going out with a senior who you won't see next year because she'll be a freshman in college. :D I'm also assuming she probably won't want to date someone still in high school and would probably break up with you towards the end of the year (that happens A LOT with freshman/senior relationships). I'm not trying to sound mean to you, it's just a reality of life.
It depends on how well you know her and how well she knows you. If you're say best friends, then that's good. :) If you only see her in the hall or maybe once during the day, it's not going to work, and you'll have to find someone else. :(
If I were you, I'd probably look for someone else in my grade level or just wait until next year. Just because you're a freshman doesn't mean you have to date someone right away. I thought I was the only one in my grade to not have a girlfriend, but that was a complete falsity. Practically no one in my grade had a girlfriend that was a senior or even in their grade level and those who were dating were either going out with someone in their grade, or a grade ahead. Now that I'm more mature, I'm ready to be dating people and am actually about to ask a girl I like in my grade that I spend time with, so maybe it's just in your head that you need a girlfriend when you really don't.
It's okay to like a senior, but just realize that it's probably for the best that you don't go out with her, unless you know she likes you too and would like to date you. Like I said, I'm sorry for your little situation and try finding someone else in or around your grade level that fits your personallity better. :)
Good luck to you; I hope you find someone soon!