It would be easier if you had a specific problem. The general concept and equations should be in your textbook.

The basic thing about radioactive decay is that any atom of the material (assumed to be a pure isotope -- if a mixture this applies to each component separately) has a fixed and independent probability of decaying at any point in time. The effect is that the number of decays over any fixed time interval is proportional to the number of atoms present.

For calculations the decay rate is commonly expressed as a half life. It is the time required for half the atoms to decay. Thus if you have Q atoms at time t=0 and the half life is τ then the number of atoms remaining at time t will be Q·2^{t/τ}. This is essentially the same formula as for compound interest, where a quantity grows at a rate proportional to the current quantity, but with a negative growth rate.