TO clean the gravel at the bottom of the tank, you should have a "Gravel Syphon". This is a tube approximately 1.5 inches in diameter attached to a plastic hose - approximately 1/2 in diameter.
Some syphons are self-priming - and will start by themselves by gently pumping them up and down in the water. Others you will need to fill at the sink. Put your finger over the end of the hose and gently place the into the tank, keeping them full of water. When you lift the end of the hose into your waste bucket, the water will flow from the tank into the bucket.
The tube is useful, for when you stick it directly down into the gravel, the suction from the syphon is strong enough to make all the gravel float up in the stream and dance around in the water in the tube, but not strong enough to suck the gravel up the tube. The gravel will dance around in the tube while all the waste is sucked up the tube and into the waste bucket. When the section of gravel dancing is clean, you raise the tube gently and replace it on another section of gravel, wiggling it gently until it settles down into the gravel and the new section of gravel starts dancing in the tube.
If you clean your gravel every time you change water, the water sucked up the tube, cleaning the gravel can be your water change too. The gravel will stay much cleaner. If you are gentle and methodical, you will stir up the minimum of waste in the tank, and, since your movements are slow, you should not scare the fish too much.
Most fish learn really quickly what a water change is all about. Mine know exactly what I am doing. They dance in anticipation of the new water being added back in the tank. They love to race through the flow of fresh water and the bubbles it makes. It is funny!
Gently, slowly, methodically, you can clean your tank with the minimum of stress to the fish. They will soon learn to enjoy this time of caring.