If you want to ad a valve and/or a secondary connection for a bidet then you will need to shut the water supply off to that pipe. The pipe is PVC, you can use a glue on valve or a push to connect valve.
It may be a push to connect fitting and if so removable without cutting the pipe, if not you need to cut it off just below it to leave as much pipe above the floor as possible.
Once your new valve(s) are installed you can turn the water back on, but if the supply lines have not yet been connected to the valves make sure they are in the closed position.
You will need a new supply line to go from the new valve to the toilet. (and of coarse a line for the bidet.)