As well as the receiver, you probably have a transmitter. Does this have sticks on it, or a wheel?
Either way, the transmitter will have several controls that you operate, to make the boat turn and start/stop. Each one of these controls has an associated 'channel' on the receiver. You should decide what control you want to use for the throttle (for a stick controller, usually channel 2 or channel 3) and then plug the speed controller into the corresponding number slot in the receiver.
The R7EH has numbered channels and a sticker on the top – the plugs from each servo/controller go in vertically, with the black wire at the bottom.
Here is a manual for a typical Radiolink – the T4EU **LINK**
and this page shows you where to put the plugs… **LINK**
If you need any more help, a photo of what you are trying to do is always useful…