What Kind of interest payments will I receive if I buy a Treasury Bill?

T-bills are issued at a discount and the difference between the purchase price and maturity/face value is your accrued interest.
Interest will effectively be paid only when the bill matures. At that time, you are given the full face value. There is no monthly interest as such.
For example, if you buy a Treasury bill for LKR 1mn face value and only have to pay LKR 900,000 as an investment value, the LKR 100,000 difference is the interest that was made on the treasury bill which you will get at maturity.