Cucumbers contain a chemical called cucurbitacin. This is what sometimes gives them their bitter taste. It is found in other fruits and vegetables as well. Bitterness in cucumbers is more pronounced when the plant is under stress. Stress may include low levels of moisture or too hot of weather. The best thing to do to remove the bitter taste is to remove the stem and peel the skin.
There are some cucumbers out there for sale and they are "bitter free" so you should try to find those. They are also called "burpless" cucumbers.
You may also try drawing out the bitterness of the cucumber by slicing it, then salting the slices and finish by draining the cucumber before adding it to your salad.
In our own garden I've noticed that cucumbers tend to become bitter when we don't water them frequently enough. They seem to be a more high maintenance vegetable than most of the other things we grow.
Several things can cause a the chemical that makes them bitter (usually this stays in the leaves). Inconsistent watering, temperatures that are too hot, and extreme temperature fluctuations over a period of time can all produce the bitter chemical. Growing your own and being mindful of this could avoid the problem.
Peel the bark, and piece them into slices. Slices should be thinner, and add a spoonful of salad cream and then, enjoy it. It would be more interesting if you add a little amount of tomatoes and chicken pieces, but if you only want to go with cucumbers, then just try salad cream.
Taking care while growing cucumber to ensure that these are not bitter is different for removing bitterness from the cucumber after these are plucked from the garden.
These days in India the cucumber we buy, rarely turn out to be bitter, but this was not the situation about 50 years back when I was a young boy. At that time a regular practice adopted by people to get rid of bitterness in cucumber and some other vegetables like ridged gourd was as follows. Cut a thin slice of cucumber from one end, sprinkle some salt on the exposed surface of the cucumber, and rub the pieces against the exposed par of cucumber for a minute or so. Repeat the process with the other end. I believe this is very effective way of removing bitterness so cucumber.
To get rid of bitterness in cucumbers, first recommended is peeling their bark. If after this step, they also have a bitter taste, left them a few minutes in milk.