First of all, you have to remember your goal. From the day you gave birth to this child, your goal was to shoot that arrow-child at the target of God's will. You wanted them to leave your home and release them as well-adjusted adults who would impact the kingdom of God. If you didn't have that as your goal, you need to start over. Just kidding!! If that wasn't your goal, you need to make it your goal now and begin praying for that to happen.
I highly recommend before you send your son or daughter off to college, that you bless your child. Have them sit in a chair as you and your spouse speak a blessing over them. Tell them about the greatness you see in them, how you expect them to fulfill God's purpose for their life, and then pray over them. This could be as simple as the two of you doing that or inviting close family and friends and allow everyone to participate. It may feel a little awkward at first, but I can promise a parent's blessing is so important and most kids soak it up. (By the way, you don't have to wait until they graduate to do this and you can do it lots and lots of times!)
What I wish we'd done when our kids were 10-years-old was begin preparing them then for what college would look like for them. (Lot of help that advice does now, right? And...planning ahead and preparing your child on everything is key!) Honestly, we didn't think that far ahead and didn't know what it would look like ourselves. We were not financial planners. There's nothing wrong with expecting your child to pay for their own schooling...it just helps if they know that way in advance. I'm not a big proponent for student loans--we paid on Andy's a few years after we were married. It's just hard starting adult life with debt hanging over your head. But sometimes, that's the only way to do it. Check with Dave Ramsey...he knows a lot more than I do about the subject.
Allow your kids to attend things like Super Summer or specialty camps on nearby college campuses while they're in high school. They'll begin to feel comfortable at certain schools and will consider attending them when they're making that choice. Our kids were on several campuses for Super Summer. It helped prepare them to know what a dorm looked like and how it felt to stay there.
Hopefully, your child has held a job at this point or has at least had to deal with high school counselors on their own. It's so important in building their confidence in being able to deal with adult issues on their own before they go off to college. I just don't think it ever hurts for a student to have a job. I carried a very full load and still worked while at college. If you look at people who've done that, you'll see some pretty determined folks. If some students have too much time on their hands, they tend to find trouble. Not all...but a lot. I'm a firm believer that working builds character and confidence.
PUBLIC VS. PRIVATE
Honestly, public vs. private boiled down to expense for us. I know from our days as youth minister, so many parents sent their kids to a Christian college thinking their child would be "safe" there--morally, and in every other way. Sadly, just because a college has "Christian" or a church denomination in front of it's name, doesn't make it a godly place to be. In fact, sometimes it's just the opposite. Because so many people are sending their troubled kids to a Christian school for that same reason, it has the opposite effect. We found that kids are going to find whatever it is they go looking for. If they're looking for trouble, they'll find it at any school. If they're looking for a Christian influence, they'll find it at the Christian campus ministries. Our kids attended a state school where there were great campus ministries. They even had opportunities to lead in them.
Whatever rule you set for grades, make it fair and make it ahead of time. They need to know about dropping classes if they're failing before it affects their grade point average. And they need to know from you when/how many times they can do that. If you expect them to graduate in four years, tell them now. If you're willing to pay for a bachelor's degree which takes them six years--while they discover what they want to be--that's up to you. But I know some parents have set a limit and told their kids if it takes longer than that, they're responsible for the finances. I believe boundaries are healthy and motivating.
So many kids pursue a major and then hate the job that degree plan produces. Encourage them to finish the degree they start. My guess is 75% of the adults aren't in the fields they studied for. (I googled it...and I was right!) I believe God will use whatever they study. Andy's degree is in Radio & Television...but he's used those skills in every church we've been in. If they really don't have a clue what they want to pursue, encourage them to get an associates degree at a junior college or go to a trade school. I just don't think you'll ever go wrong knowing how to do things with your hands.
What about those kids who don't want to go to college? Not all of our kids did. Our philosophy was, "That's fine. But you have to get a job. You will be a responsible adult."
What happens if/when your adult-child messes up? Allow them to suffer the consequences of their choices now. If you don't, the consequences can get much bigger. The goal is for your child to leave home and make good decisions. They need to know you love them and that your door and hearts will always be open to them, but you're not going to rush in and rescue them now. They are adults. But if you're going to tell them they're adults, you have to be willing to treat them like adults. That's the hard part.
Finally, I hope you've taught them not to mix whites with reds while washing clothes...and not to put aluminum foil in the microwave! With those tools, they should be good.