How to link combo boxes and list boxes in VBA using active x controls

Okay I will start this off by saying that I am very new to coding in general and just learned what VBA was 2 weeks ago via my professor who has showed me very limited. I honestly at this point do not even know why we are using active x controls, but I know he told us to always use them instead of form controls. I have to write a whole program with equations that are dependent upon what gas via "combobox" then it corresponds to a critical temperature and critical pressure into two separate text boxes. I am trying to do all this via userform without inserting a module because he has never shown us modules and I have no clue how to link the program of equations I have to write within the userform into the module. I also have to import the data from the gases via spread sheet.

example: temp pres
Column A column B Column C

methane 56 110
ethane 76 120
propane 85 105

how do I write the program with a command button within the user form that way when I choose methane it displays the temp and pressure in a text box.

All the help I can get is greatly appreciated. I know this is probably basic stuff but to someone who knows nothing about it, it is very hard to figure it out. I have been on the web for hours looking.

Thanks so much that is exactly what I needed! I knew it had to be simple. Is there any good books you would recommend to learn VBA?