Q. Should there be a comma after “also” when it begins a sentence?
A. Yes, an introductory “also” would normally be followed by a comma. The relevant rule is the one that applies to an introductory adverb like the word yes in the previous sentence (see CMOS 6.34). Note, however, that after an introductory adverbial phrase rather than a single word, the comma can often be omitted, particularly if the phrase is short (see CMOS 6.31). So,
As of late 1999 our Y2K fears still seemed warranted.
but
Also, many of us were new to computers back then.