Where do you plug in speakers on your dell computer?

"The green plug" isn't exactly the most descriptive term.

Speakers normally plug into the speaker port. This is probably identified on the case with a symbol that looks at least somewhat like it might be a speaker. A trapezoid, or something that looks like a set of headphones, or a series of concentric arcs are all reasonable possibilities. Alternatively, it may be labeled "spkr" or something similar.