Discuss everything from the very beginning
It might take you and your flatmate a while to get used to each other, and it is important to know how to behave towards each other from the very beginning, conflicts and mutual resentments are not the best way to figure it out. It is natural that you will have differences in views on what is dirty, what is clean, what is loud, what is quiet, because you grew up in different families, you are used to different lifestyles.
On the moving day try to think not only about how you will unpack and decorate your room, but also make time for conversation with your new flatmate. Do not miss the moment to agree on the rules in your common home. Much depends on how you will start your coliving.
Discuss the cleaning schedule. You should decide together not only how often and in which order you will clean, but also agree on the expected volume of cleaning. Someone is used to washing dirty plates immediately after eating, in some families people wash them only when there are no clean eating utensils anymore. For someone is enough to wipe the dust off to maintain cleanliness at home, someone vacuums and washes the floor every week. Agree on how you will take out the trash. Set a date for general cleaning, agree on who, when and how will clean the windows, defrost the refrigerator etc. You can even put it all in a special table, print it out and hang it in a prominent place.
Discuss shared costs. Agree on who and when will pay for utilities, how they will be divided, how they will be paid in case of a long absence of one of you. Make a list of the little things that may be required in everyday life (for example, matches, toilet paper, dish soap, light bulbs), and discuss how you will share these costs. For example, you could have a special box where you will put the same amount of money at the beginning of each month, then each of you would be able to take the money from there and buy such things, if necessary. To keep things fair, put all the checks in this box and buy everything you need in turn. You can also use special applications such as Facebook Messenger for cost sharing. 
Discuss when “quiet hours” will begin in your apartment. Even if each of you has private room and can close the door to it, it is possible that it will not protect you from unnecessary sounds from the common area (hallway, kitchen, bathroom).
Ask your flatmate about his/her routine, habits, inner circle, and whether he/she plans to bring guests and how often.
Just talk to each other
It is quite normal that some habits (such as listening to loud music, smoking out the window) can be annoying. Therefore, be ready not only to complain, but also to listen to the other side and to respond to criticism appropriately. Don't yell, stay calm, choose your words carefully and speak politely. But don't be silent, because other people can't read your minds. And what is obvious to you may not be obvious to others. Your flatmate may not know that he/she does something wrong. Or you do not suspect that some of your actions annoy your flatmate.
Respect each other's personal space
It is good when your flatmate is your friend from the beginning. However, it is not necessary for living in a shared flat. But very important thing is to respect each other's personal space. You should treat others the way you want to be treated. How would you like it if someone waked you up or suddenly broke into your room? Don’t be imposing, don’t be too loud, don't mess everything up.
Even if it is not late, you never know what your flatmate is doing in his/her room: perhaps he/she is sleeping, or meditating, or studying/working, or feeling bad and doesn’t want to hear your loud music all over the apartment. You don’t wash dishes, do you? Perhaps your flatmate will need them, and it will be unfair if he/she will wash them instead of you. You invited a noisy company without warning, didn’t you? Perhaps your flatmate planned to spend this evening in silence, with a book. When you live in a shared flat, try to think not only about yourself, learn to imagine how your flatmate sees you from the outside and how your actions affect him/her.
Remember that people won’t play only by your rules in a shared apartment. This will happen only in your own home. According to the study, the average age of people who could afford to buy real estate for the first time was 23 years in the 60s. Now it is 31, and the reason for this is the crisis in the global economy. [5, 6] Therefore, if you don’t have the opportunity to become the owner of an independent housingin in the nearest future, get ready to compromise with your flatmate. Do not demand a lot from him/her and respect his/her right to live in clean, safe and comfortable house.