For the most part, citations of ancient works begin with the author's name (sometimes abbreviated), followed by a short or abbreviated title, usually in italics. (In cases where only one work by an author is known, the title may be omitted.) This is followed by a sequence of numbers and/or letters that indicate the specific subdivisions of the work. Arabic numerals tend to be favored in recent publications, but older citations often use Latin numerals as well. Exactly how works are subdivided varies. Book, chapter, section and/or line numbers are often provided. Some works are cited by page numbers (and/or columns) of standard, or once-standard, editions. Scholarly editions of classical texts will be subdivided by these schemes, as will many translations. 

Abbreviations should be taken from an authoritative source such as the Oxford Classical Dictionary (see below). You should also provide bibliographic details of the edition and/or translation of the work you are using: generally when the work first appears in your notes and in your bibliography.

For more information see the Chicago Manual of Style, 17th edition (Chicago, 2017) §§14.242–252, also available online. See also The SBL Handbook of Style For Ancient Near Eastern, Biblical, and Early Christian Studies (Peabody, MA, 2014), §8.3.14.

Sources for Abbreviations

Classical Authors and their Works

• Greek-English lexicon / compiled by Henry George Liddell and Robert Scott (LSJ) (Oxford, 1996)  Online abbreviations
• Oxford classical dictionary (Oxford, 2016-) Also includes abbreviations for some periodicals, collections, and reference works.]
• Oxford Latin dictionary /  edited by P.G.W. Glare. 2nd edition. (Oxford, 2012) online
• Thesaurus linguae Graecae : a bibliographic guide to the Canon of Greek authors and works Maria. C. Panetelia (2022)
• Thesaurus Linguae Latinae: Index librorum scriptorum inscriptionum ex quibus exempla afferuntur

Early Christian and Byzantine Authors and Works

• Clavis patrum graecorum / cura et studio Mauritii Geerard. (Turnout, 1974–) 5 vols.
• Clavis patrum latinorum / commode recludit Eligius Dekkers. (Steenburgen, 1995)
• Clavis clavum (Brepols)
• Oxford dictionary of Byzantium / Alexander P. Kazhdan, editor in chief 3 vols. (Oxford, 1991)  and online
• Patristic Greek lexicon / edited by G. W. H. Lampe (Oxford, 1961) archive.org

Some additional points

• Square brackets [ ] around an author name indicate that the authorship of the work is doubtful.
• A modern name or initials at the end of the reference usually indicates the editor of the specific edition being referred to.
• A numerical superscript after the title of a work usually indicates its edition. 
• An equals signs connecting references equates multiple systems of reference and/or indicates the text in question can be found in both soutces.

The abbreviations of journal titles in classical studies and related fields found in the Oxford Classical Dictionary and L'Année philologique are widely used. For abbreviations not found in these sources, check Wellington's Dictionary of bibliographic abbreviations.