Ok, there are a few different types of BIOS and they have different ways of presenting the various details and options.
Fortunatley most have the information at the bottom of the screen to tell you how to change the setings.
All you do is highlight the bit you need to change or the screen that you want to explore and hit ENTER, its all keyboard stuff in bios not the mouse, to exit that particular screen hit ESC.
Usually the up and down keys are used to highlight the bit you want to change and the page up and page down keys change the value.
After you have made any changes and are ready to exit the bios you usually hit F10 and then Y and enter, if its one that doesnt work like that select Save Changes and Exit.
Only do this after you have made all the changes that you are making, although in this case you are only wanting to change the one thing
You are looking for a screen that says BOOT sequence and will probably be set as A,C, SCSI or A, CD Rom, C or some variation on letters, you want CD rom as the first.
Other types of bios actually have First boot device, Second boot device and Third boot device, in that case just change the first boot device to cd rom