The most valuable thing that you have to offer these students is your own experience as a college student and graduate student. This might seem surprising, but is based on my own experience of working with high school students in the U.S. Many of these students aren't sure about going to college, don't know what will be expected of them when they do go to college, and aren't sure that they're "smart enough" to succeed once they get to college. Mostly they need mentoring and encouragement more than they need instruction in any particular aspect of mathematics.
There's a good chance that you know more about what's going on in college than even their high school teachers (who may have been out of college for many years.) You're closer to their own age and not an authority figure, so they're more likely to believe what you have to say. You should talk to them to find out what would be most useful for them to know.