The Hardy-Weinberg equilibrium for two alleles at one locus is: p2 : 2pq : q2.
So what does that mean? First, p stands for the percentage of allele A in the population, and q stands for the percentage of allele a. Since the total number of alleles adds up to 100%, q is also 100% minus p. (More conventionally, these are usually written as decimals, so q = 1 - p.)
When two individuals in the population mate, there is a p% chance that the egg will have allele A, and a q% chance it will have a. Likewise for the sperm. The chance for an egg and sperm both with A will therefore be p x p, or p2. Likewise for a and q. The chances for a match of A and a, or a and A, are p x q and q x p, which makes it 2pq.
Thus, individuals of the next generation will have a p2% chance of being AA, a 2pq% chance of being Aa, and a q2% chance of being aa.