what is the best way to gain a sample from aquatic insects? please help

If I understand your question correctly, you want to have a look at insects that live in water?

We did something similar in high school. We collected pond water. If you put drops of it under a microscope you can see what's in it and maybe try to identify things that you see. Sorry I can't recommend any texts to help you with that but maybe others on here can.

Your question is a bit unclear, but if you mean you want to catch water boatmen then passing a dip net through the reed beds where they are will be the most effective method. Just tip the contents of the net into a shallow tank or tray and you should find lots of aquatic insects in your sample.