Discount Rate is otherwise known as Cost of Capital as the term is used in Capital Budgeting
Most metrics such as NPV, MIRR, Profitability Index and Discounted Payback Period use this rate to discount the cash flows.
Although I have expertise in calculating or computing these metrics given the cash flows, initial cash out lay and the discount rate, however I have not managed to delve deeply in to the way this discount rate is calculated
A good way to start is for you to look up WACC which stands for Weighted Average Cost of Capital and this is what you referred to as the Discount Rate
It is made up of proportions of equities and debts which is multiplied by risk factors and finally summed up