Are you worried that your sweet darling is going to turn on you without warning the minute puberty kicks in? I can't help you with that, but I may be able to warn you about a couple of things that may give you a slight edge in the situation.
Your 6th grader doesn't have to have instant computer communication with all of their friends. This communication does seem to be necessary when they are a bit older, but in 6th grade they need to figure out how to deal with their new life and time management. Too much unregulated access (and do you want to be the policeman?) is a giant sink hole of problems. If it is difficult for you to self regulate your time on Facebook (you know who you are), it is impossible for your child. You have the power to choose what comes into your home. I just advise that you make conscious choices at each step along the way. Consider whether it is wise to have a computer in their room. This is not necessarily a trust issue. How many times have you looked up from the computer and it was three hours later and you had no idea that time was passing (I am doing it right now!). The time will come when they will go to bed after you are long asleep, but right now they are the ones that really need the shut eye. They are so much nicer when they get it.
That brings up the fact that they look old, but in many ways they still have the same needs that they did when they were toddlers. Sleep, eat and read aloud should be your mantra. They grow when they sleep and they are growing inches every day. Studies show that US teenagers are chronically sleep deprived. I am mean when I am tired (really, don't cross me!) and they will be too. Why go there?
Feed them right away when they get home from school, something healthy with complex carbs or protein. They probably had lunch at 10:30. They are HUNGRY. Feed them before you ask about their day or they will bite your head off.
6th graders have a lot to deal with (more on that in the next "preparing..."). A funny thing happens to many of them. They occasionally take up a little baby language, they climb their giant 5' frames onto your lap, they form a sudden nostalgia for their "childhood". While they are desperately trying to be older, they are also uncomfortable with letting go of the familiar. This is why it is nice to stay friends with kids that are not going to your middle school. They have to be cool with their school friends, but they can be their old selves with their old friends. Don't get rid of all of the Legos or stuffed animals just yet.
They may not want to talk about their day when they get home. A parent can look pretty lame in the cold light of day, but at night, when they are cozy in bed and the lights are low, you look like their beloved moms and dads again. Everybody likes to be read aloud to. If you keep doing the bedtime reading (or start back up) you will be amazed at what happens. First the books just get better and better. But you may find that you don't spend much time on the books. You may end up just talking, and the longer you can keep that tradition going the better.