The speed of the boat depends on who is skiing and what type of skiing you are doing. The main thing you want to do is maintain a minimum speed to keep the skier upright. If you go too slow, then the skier is not going to have enough momentum to stay upright. If you go too fast, then the rider is going to be knocked off their feet by hitting the wake of the boat.
A lot of the speed depends on the weight of the rider. A child on two skis would need a speed of around 25kmh to stay upright. An adult on one ski would require a speed of around 58 kmh. If a skier is barefoot, then they may need speeds of up to 70kmh. Start slow and work your way up to a constant speed where the skier is able to stay upright easily.
If you are entering a competition, then for slalom skiing, the speed can be anywhere between 22 and 58kmh. For speed races, the speed can get up to 190kmh. Always remember to wear a helmet.