When you write a paper, citation styles require you to cite your sources both in the body of the paper (abbreviated citations) and at the end in a reference list (full citations). The ACS citation style has 3 citation systems. You can choose from 2 of these for your assignment - the author-date or numeric superscript systems.
Author-date in-text example: The primary structure of this enzyme has also been determined (Finnegan et al., 2004).
Author-date reference list: Citations are in alphabetical order by the first author's last name.
Numeric superscript in-text example: Oscillation in the reaction of benzaldehyde with oxygen was reported previously.3 (pretend the number is smaller and higher)
Numeric superscript reference list: In numerical order according to superscripts in text.
I highly recommend using the author-date system if you are going to do your citations manually. If you use numeric superscript, any time you change the order of the in-text citations, you'll have to reorder the reference list, which is a drag. With a reference manager, you can do either system because both are available in the citation style choices.