Many people from different countries have improper intonation because their teacher had improper intonation. In many situations, being monotone is better than having bad intonation. The worst is when someone goes up and down too much on every word. Another thing you should avoid is to end high at the end of a sentence.
To correct improper intonation, you need to remember to start high and end low. You cannot do it any other way. I recommend going to the interview section and listening to one of the audio files provided by a native speaker. You will hear proper intonation. After listening to the audio, record yourself and listen to it. Does it sound the same? If not, then find the areas that are dissimilar and make the necessary corrections.
This advice is very trivial, but intonation is relatively easy to correct. You can fix your intonation with only a little effort. If you have a friend who is a native English speaker, you can usually fix intonation in a couple of lessons. Recording yourself and listening to yourself might be tedious, but having correct intonation can go a long way.
I have had many students who fixed their intonation after 2 small sessions. The advice I gave them was the same as the one above. This will work, and if you want to speak with proper intonation, please follow this advice.