I would tell them that if they are getting a liberal arts degree it really does not matter what they study. They should pick something that interests them because they will have to be immersed in that for four years. Additonally, they should consider what type of experience they want to gain. It is important to know what size of school you want to attend, if you want a religious institution, and what kind of location you want to be in. If you know that you want to get off capus and do other activities, you should not apply to a school in the countryside.
Don't be scared. Never be scared. Whatever it is, wherever it is, jump in, and get your hands dirty. College is about new experiences and new opportunities, take advantage of them. Never again in your life will you live with the same kinds of people as you will in college. Never again will be able to change so much, so quickly. Parents, whenever we come home, accept us. We might change, but we still need your support and love. We want that, more than anything. Sometimes we're afraid that we are disappointing you. Let us know that we are not, okay?
Make sure to do overnight campus visits! This is a great way to get the temperature of a campus beyond just taking the campus tour or visiting classrooms. In conversation with students you can get a feel for what happens on an average weekend, what political and social life is like, and where you might fit into the campus as a whole. I was a little bit nervous about doing these visits as a high school senior and was forced to do them by my parents, but they ended up being the most helpful part of my decision-making process.
When you choose a college, you should consider practical issues such as academic reputation, tuition/financial aid, campus environment, etc. However, the important thing is your attitude once you get there. Don't assume too much. You will find that reputation isn't everything- sometimes the weird people make the best friends, and sometimes the presigious professors are lousy teachers. If you've never dated before, don't start your freshman year.
don't put off what you need in regards to making your application quality. build relationships with those whom you will be able to request recommendations for your application. visit colleges and universities that you are keeping in mind. take into consideration what you want and need (location, distance from home, financial aid, tuition, room/board, majors) plan, plan, plan. this will save you a lot of time and money from being wasted.
Don't choose a school based solely on how it will look on a resume. Take the time to find a school where you will enjoy not only learning, but also day to day living. A good college experience is more than just the value of an education. College is a time to really craft individual identity, and picking the right school is an important part of this process.
If I had the ability to talk to myself as a high school senior knowing what I now know I would tell myself to be prepared. I should have applied for more scholarships and put myself and my academic acheivements out for more people to see. Then perhaps instead of paying off loans I could be saving that money up for something realistic like a house.
Take the process very seriously; don't make some glib, spur of the moment decision based on where your girlfriend/boyfriend is going (or best friend for that matter). Make the decision that suits your needs best and make the decision very carefully. There is no feeling worse than being at a school that you absolutely hate.
One size definitely does not fit all. If your college experience somewhat resembles my experience it will be filled with many of the greatest memories of your life, but also some of the worst. Know yourself before applying and weigh your strengths and weaknesses with the schools to find a great match.
Make sure to find a college that fits your personality best. Don't just base your decision on what kind of academic standards they have. Make sure that you are comfortable there and that there is room for fun as well. You want your 4 years of college to be a good experience.