You may desire to include content in your paper that includes in non-Roman scripts or to insert non-alphabetical characters such a symbols. Below are a few resources to help with this.
Do bear in mind that ACM’s publishing systems do not support every possible LaTeX package. A list of accepted packages is given here: https://authors.acm.org/proceedings/production-information/accepted-latex-packages. So, if adding a special character that requires use of a package, you are advised to check with this list first. If the package you need is not supported, either use a different method or seek further guidance: bear in mind, there may be more than one method to insert a given character.
Before adding new packages, it is a good idea to try adding a ccharacter, using the suggested code, and seeing if it works. If it does, the ACM’s acmart package likely already includes that package so no further modification is needed. Be aware that a number of packages are used internally by the acmart package but that they are not formally listed in the above page. Thus the suggestion of a first check without adding any package references.
Overleaf
Even if using your locally installed LaTeX, Overleaf’s guide on adding characters is a good place to start, as it covers many of the commonest scenarios. See Overleaf’s list of Greek letters and maths symbols and remember to check the ;ist of further links at the bottom of that page.
The Comprehensive LaTeX Symbol List
A fairly exhaustive listing of symbols that may be added in LaTeX can be found at https://tug.ctan.org/info/symbols/comprehensive/symbols-a4.pdf (481pp, c.32MB). Each section describes a set of characters and the packages and/or extra setting needed to use those characters. Do remember to check if extra packages are supported by ACM (see above).