The trick is to turn. When your skis are pointed across a hill (and on the uphill edge) you don't accelerate. Point them just a tiny bit down and you start to slowly move. Point them straight down and you go at maximum speed.
A good trick to build confidence is to practice "J turns".
Find a nice easy slope to practice - think about which way you feel best at turning (everyone usually has an easy side and a hard side), and point down the hill - this is the straight part of a "J" and then once you are sliding, turn in your favourite direction until your skis are pointed across the hill. You will eventually stop!That's the basics of what any advanced skier does, they just do it very efficiently.
Of course, once you get comfortable, stop turning halfway through the J turn and turn back the other way, soon you will be making linked "s" turns down the hill. If you ever feel unsafe, just turn across the hill until you stop.
That's the trick!