There are two parts to your question: a) defining a relationship that matches the records in one table to records in another table by room. A room name field could be used, but a room ID field is a slightly better option. (If you remodel or redecorate and your rooms are given new names, you don't want changing a room's name to disconnect it from related records in another table.)
b) is the main part of your question, setting up a value list of those room designations so that you can select a room for a record in your second table in order to link it to a room record in your table of rooms.
for b) Open up manage | Value lists and create a new value list. Select the "use values from field" option and select your table of rooms from the drop down list in the next dialog box to appear. If you stay with room names, you need only click to select the room name field from the list of room names that appears below the drop down. If you want to associate a room name with a room ID, select the room ID field in the first list, then select "also use values from" and then select the room name field from the second list. This gives you a two column list where you can select a room by name, but enter the room's ID into your field in order to link it by Room ID to a record in the rooms table.