In order to attach the floor joists correctly, you will need to improvise and attach a ledger board to the wall. Secure it to the wall using nails and 4-inch lag bolts. They should be driven into the wall studs supporting the wall. Once the ledger board is secure, attach joist hangers and fasten them to the ledger board. Space them appropriately, depending on the spacing of the floor joists. Measure them precisely just to be sure about your spacing. Once you have secured them to the ledger board, you will now be able to fit the floor joists into the hangers and the wait of the joist will be transferred to the wall.