As others have said, the answer to the question depends on the company instructions, the type of stock, if it's an in service passenger train (with guard) or ECS / light loco or similar, if the station is staffed, the local instructions that relate to that station (see sectional appendix, local instructions section) and what signalling equipment (if any) has been provided at that station.
At Bristol Temple Meads when I was there, all in service passenger trains were dispatched by the station dispatch staff indicating to the guard. The guard then used the buzzer to communicate to the driver that the train was ready to depart. The R.A. was normally only used for ECS / light loco etc.
T.R.S. or T.R.T.S. is a system whereby station dispatch staff can press a button on / in a device mounted on a wall or post. When the button is pressed, an indication will be shown in the signal box / PSB / signalling centre so informing the signaller that the train is ready to start (depart). The indication extinguishes automatically when the train leaves the platform.
R.A. is a system whereby station dispatch staff can press a button on / in a device mounted on a wall or post. When the button is pressed, if and only if the relevant signal is showing a proceed aspect, on the same signal structure a "RA" indication will be shown to the driver. This informs the driver that station duties are complete and that the train may now depart. The R.A. resets automatically when the signal goes back to red.
As others have said, get advice from your trainer / instructor.