Learn English – say “I sent her a message about”

As Tyler James Young says in his comment, it would be more common to ask, "How did you get here?" rather than come, and starting your response with the rhetorical question "How'd I get here?" is perfectly natural in casual conversation.

If you were talking about a position/status in life, you'd say something like, "How did I come to be the vice president at the age of 28?" and I think "How'd" would work in informal conversation. If you were talking about a place where you reside (rather than a restaurant or a party, where you recently arrived), you'd also use come to as in "How did I come to live here?"

Since the question is rhetorical (I think that's the proper term, since it's a device rather than an actual question for your listener), "How did" sounds like it's the best overall answer. The dynamics of starting your answer to a question with a question feels more comfortable with a longer answer.

Well, the first thing I must point out is that neither of these sentences are correct without an 'a' in them.

It was a pleasure to meet/meeting you.

As for whether you should use "to meet" or "meeting", it makes no difference. The meaning and usage is completely interchangeable.

I would not recommend using "to have met you" except for specific situations (like thanking a host). The meaning is subtly different. The first sentence means "I enjoyed meeting you." But the second means "I enjoyed that I got to meet you." It doesn't say anything about whether the actual meeting was a pleasure, although this is implied.