'Since' and 'From'
'Since' talks about one particular point of time till now.
I have been working since 7 am
'From' in most of the cases talks about the starting point and ending point. Said that, the format could be 'from....to/till/until'
The shop is open from 9 in the morning to 6 in the evening
Quite close is 'since' and 'for'
'Since' and 'For'
Okay, as I said, 'since' talks about a particular point. So, you need to mention "since" which date the thing has been happening.
On the other hand, 'for' is used for a total period of time and you don't need to define a starting point.
The best example I can think of is...
I have been waiting for two hours
but...
I have been waiting since 2 o'clock
In the latter example, you need to define the starting point.
Another example:
I have been living in this house for 10 years
I have been living in this house since 2005
So, in your case, you have known him for the past 15 years OR since 2000.
Let's get the easier question out of the way first. When you are confused, you can use about or on, but I think about is the "safer" choice. Behold the ngram.
As for for the taking, let's say that you need a job, and I own a business. I might tell you:
I have a job opening. It's yours for the taking.
or:
I have a job opening. It's there for the taking.
Basically I'm telling you that you could get a job with me, if that option sounds good to you. I could easily substitute "if you want it" for "for the taking".
Looking at the ngram, we see that common ways to use the phrase include
- there for the taking
- free for the taking
- [possessive pronoun] for the taking (i.e., his for the taking, yours for the taking, etc.)
I won't say those are the ONLY way the phrase can be used, but an answer can't delve into every possible usage of a phrase. Those three are a good start.
There's also a relatively high number of
- compensation for the taking
but those come mostly from legal documents; that's not a wording generally heard in everyday conversation.
Best Answer
The preposition 'on' is correct.
We use 'on' for most of communicating devices that connect
However, for computers, remember one thing.
When it is general and not so specific about the location, we use 'on'. So, it is broadly talking about being held by the computer.
But then when you go further, one step further for a specific location, we use 'in' the computer.
You may also use 'in' when referring to an item that has some spacial boundaries.
Things without specific boundaries
As James says in the comment, making folders on some drive is also used but it is a bit more subjective.