How to hook up my 5V water pump

This is project number 1 so steep learning curve ahead. I thought Id throw myself straight in and make a little moisture controlled water pump. The sensor work fine and all is well although I cant get the pump to work (its via a relay). My issue is that I dont know how to ground the pump mainly because I dont understand the join on the diagram. So basically where does the black wire on the water pump go?

First get the model and datasheet for the pump. The pump most likely uses a motor which means inductive emf and higher current draw. You should be supplying separate power to that motor. Not sure why you have the pump in series with the sensor this is not the way to use the relay. Put the sensor on it’s own and isolate the relay to switch only the pump. Give the pump separate power. That relay module is designed with what you need to keep the pump circuit separate from the low voltage Arduino circuit.

All motors will take more current that an Arduino can safely provide. Keep motor supply separate from
Arduino supply, though you can common the grounds (not needed if using a relay). Expect your motor to
require several amps at least of stall current. The supply for it must be able to handle that without cutting out.

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You have not shown any power supply for the pump.

An Arduino does not in any way represent a power supply. You need a power supply for the pump. If the pump runs on 5 V, then you need a 5 V power supply rated for whatever the pump requires. Do you know what the electrical specifications of the pump are? You are looking for its "stall current".

Once you have that power supply, then you connect the pump to the power supply with the relay contacts in series with - generally the positive lead.

get a second wall wart, put the relay power and the pump on it.
use the first power supply for the Arduino board.
use a 330 or 500 or even 1k ohm resistor on the output.
tie the grounds together

everything should work as expected.