It could be said that the choice of a college in which for the student is an emotional one and how to fund it is less so. For the parent it is the opposite, the paying for college brings about quite a bit of emotion (i.e. concerns on how to pay for this an retirement, guilt for not saving more, etc) and less emotion on the selection of the institution. This is of course an oversimplification since both parents and students use a combination of logic and emotion when making decisions when it comes to college. Communication for this process is important so don't delay and consider making it a formal 'talk', one in which is put on the calendar and make it a regular meeting. This allows the student to see that it is a very important topic and that you consider their input vital in the process. This ensures that the student doesn't feel like it's like the time you told them they couldn't go somewhere for a particular reason and no discussion was needed. Instead, this is a process that you are working on together. You still have the final say over your finances but a good decision is one that all parties are in agreement.
A good way to start the conversation with your student is to simply ask what they expect to get out of college. Have they thought about a career and does the school offer a good degree that will point them in the right direction for this career? Does the career fit with their personality and interests? Consider including in the discussion the idea of going to a local community college. This could especially be a good move if the student does not yet have a direction that they want to go in. If they have a college or a few in mind, ask them why they appeal to them. At this time, you may want to suggest they cast a wider net and consider other colleges. This may help them better know through their research that their 1st or 2nd choice colleges are definitely the right ones. The other benefit is that it may help lower the cost on their selection since having competing offers from some of these institutions when and if you try to appeal the amount of aid they will give.
Discuss what your and the student's expectations are when it comes to how much the student has 'skin in the game'. Will it include part time or summer work for income. Will it include student loans, if so how much? This is where you can request the student to fill out a budget for the time during school and one once he graduates. Be sure to include student loan payments as possibilities. You can bring up that the need for student debt may be higher if they attend one college over another. Be sure to take this opportunity to make sure they know how loans work, how much their payments may be etc. You can also make a stipulation that any college expenses after four years are their responsibility or at least some consequence so they know they need to be a good student while in college.
This may also be a good time to mention that you also have to plan for their brother or sister if they have them. You can let them know that you want to help them get to the college that is best for them but the reality is that the amount of money spent for one student may adversely affect their brothers or sisters who also intend to go to college and require resources as well.
Never forget that while discussion about the finances around this may at times feel stressful, you can pause and remember to see this process as an opportunity to strengthen the relationship with one another. The student is heading into a time in which they will be more independent. While understanding this, parents want their child to know that they are there for them. Ideally, they want to be someone the student can talk things over with not just for today but for the future as well. The student and parent can come out of this with the beginning of a more 'adult' relationship that can pay emotional as well as financial dividends for quite some time.