Comma after also at the beginning of a sentence

"Also" followed by a comma, indicating an intonation break, tends to be interpreted performatively, but without a comma is interpreted declaratively. A performative concerns the saying of something, while a declarative concerns what is said.

"Also," at the beginning means, roughly, "in addition to what I have just told you, I am telling you what follows after the comma." But "also" not followed by a comma means that what follows is a factor in addition to those previously mentioned.

Other adverbs in initial position have potentially both declarative and performative senses which can be disambiguated by making an intonation break after an adverb intended performatively. For instance, compare

"Frankly I spoke with her."  
"Frankly, I spoke with her."  

The first means I was being frank with her. The second, the performative, means that in saying that I spoke with her, I am being frank with you, the person I am now speaking to.