When should I capitalize people's job titles?

If you mean the abbreviations of job titles, such as Mr. , Dr. , Prof. , MD , the initials are always capitalized. But, if you mean the job title, such as English Teacher, Civil Engineer, etc. ,I think, when you use it on a business card, a resume, a formal document, etc.

As customary, a job title is capitalized when by itself such as on a business card, a resume or at the end of a letter with the signature. If within a text usually no, unless it's an acronym (CEO) or abbreviation (Mrs.).

by their desginestions. and if used Mr. or Mrs. or Miss.. always capitalize the first letter.

with the abbreviations , I guess

when you use it on a business card

I do that only with CEO or Top management