I have to agree with the others about external speakers being a good choice.
That said, if you are really deadset on opening the iMac yourself you will need to be a bit more specific as to which machine it is. There have been 10 years of iMac revisions and the method of disassembly varies.
Where did you get the replacement speaker? If it is specifically a replacement part for the broken one did it not come with any instructions for installation?