An English question: When do we use "how to"?

Both are one thing,and true.

Yes/No questions are the most basic type of question. You can use them to ask for a simple yes or no answer. They usually begin with a verb, including auxiliary verbs (a “helping” verb that comes before the main verb) or modal verbs (such as can or would).

A) would not really have a question mark (but it's possible) : this HEADING would precede instructions 

For example : How to beat Chelsea :-

1. Keep the ball

2. Play safe...

In a newspaper, 'How to beat Chelsea ?' or 'How to beat Chelsea' could be headlines or smaller headlines 

Or parts of an actual conversation or parts of an article

B) is either a heading with a question mark OR a question

For example : 'How do you beat Chelsea ?' as a magazine headline

Or a sentence in a conversation or a sentence in an article

Check out PC being a personal computer too

strictly speaking, you need to use a verb somewhere to make it a sentence.  This is why most would say "how do you start" instead of "How to start".  How to start can be used as part of a secondary clause (how acts similar to "and" or "but"), or adverbial clause, as in "I want to know how to start..."  Sometimes, people don't say the actual words "I want to know", but they are implied if not declared.  It is only acceptable to omit the main part of the sentence (the actual subject and verb, as in "I want to know") in situations where it is extremely obvious and time or space are important and the use of a fully grammatical sentence is not all that important (like as part of an informal conversation or as input for a google search).

B is correct, but many people on Yahoo! Answers format their questions like A.

Generally speaking, not as a question. "How to start a PC" would be a title or something you'd say before or in relation to providing instruction on how to start a PC.