So if, say, a fair coin is tossed three times, the probability of a head or tail will be equal (50%).
At least one head means that out of the three trials, you will get a head at least once. You therefore need to consider all possible outcomes: three heads, two heads and one head. That will be P(HnHnH)+P(HnHnT)+P(HnTnT). I'll assume you know the multiplication and addition rules.