Is it true that plants release CO2 during night and that I should not have one in my room?

You are correct, plants do release carbon dioxide (CO2) at night, although they also release CO2 during the day.

This is a part of the process of respiration!
First though, you should know that during the day, when there is enough sunlight, plants undertake a process called photosynthesis. This is where they use CO2, water and sunlight to produce sugars to be used as food. Since plants can’t really move around to get food, they need to be able to make it themselves (see my previous answer if you want to learn more about this process). 

To use the sugars that plants make through photosynthesis, they need to convert the sugar to energy. The process that plants use to do this is called respiration. Respiration releases energy from the plant’s sugars and as a part of this process the plant releases CO2 and water. So this is where the CO2 comes from.
You can do a little experiment at home if you would like to see plants carrying out respiration in action!  See the experiment at the bottom of this link for more information.
 

Unlike photosynthesis which can only be carried out in the green parts of the plant, like the leaves and stems, respiration can occur throughout the plant. It is also a process that is happening all of the time, so during both the day and the night! Please do not worry though, although plants are releasing CO2 at night, it is not enough to be harmful to any people also in the room. Humans also release CO2 when we breathe out, one plant is not going to release anywhere near as much CO2 as one sleeping human. If we can safely share a room with our siblings, we can definitely share a room with our plants!

Plants are actually very good for our health and well-being. It has been shown that contact with nature can help to reduce negative feelings and boost positive feelings. Caring for plants can be very relaxing and it can be a rewarding experience to see something that you look after grow and thrive. Because of that, I would recommend having a plant in your bedroom. I have many in my room! I enjoy spending time watering them and getting my hands dirty when they need some new soil. Succulents are a kind of plant that can be an easy option to look after. They do not require much watering, but when placed in the right spot will grow very well.