@Suhail
It would work differently on mobile app and differently on laptop or pc. depdning upon how you wish to access your room, you can on your pc or laptop first chose the room by going to your default room first and then clicking on the room icon in the bottom of your screen and list of rooms will pop up in which you have click on the 'i' ( information button ) in the respective room and then select to make it default room. Once you select the room you have as your default room, you will be able to use it from your mobile also subsequently. You can also use the rooms of your choice in mobile also but you have to login to imvu next web page from your mobile through the browser and open it in your mobile client.
Hope this helps to some extent

bet of luck