How can I decide when to use “for” + “-ing” or “to” + [infinitive] in a sentence?

Example:

I have nothing to do for now.

Nothing for doing for now.

Another one:

I have some things to study.

I have some things for studying.

New Examples: (QUESTION UPDATED)

I don't know if you use XXX, but I was wondering if it could be useful for sharing knowledge among us.

I don't know if you use XXX, but I was wondering if it could be useful to share knowledge among us.

Another one:

So, we could create a group to share any kind of technology knowledge.

So, we could create a group for sharing any kind of technology knowledge.

Answer

This is a very good explanation of the distinction; I have included it reformatted below:

We use for + the -ing form of a verb to talk about the function of something or how something is used:

I need something for storing CDs.

The PC is still the most popular tool for developing software systems.

We use for + the -ing form of a verb to refer to the reason for something:

You should talk to Jane about it. You know, she’s famous for being a good listener. (A lot of people know she’s such a good listener.)

Warning: We don’t use for + -ing to express our purpose or intention. We use to + infinitive:

We’re going to Lisbon to visit my aunt.

NOT: We’re going to Lisbon for visiting my aunt. or … for visit my aunt.
He’s now studying to be a doctor.
NOT: He’s now studying for to be a doctor. or … for being a doctor.
There’ll be sandwiches to eat and juice to drink.
NOT: There’ll be sandwiches for eat and juice for drink.

I am, however, inclined to disagree with the phrasing of the warning a little, and would phrase it as

Warning: We don't use for + -ing to express an aim or intention. We use to + infinitive.

"Purpose" is a poor choice of words to use in this warning. As you might notice in the first example that this passage offers,

I need something for storing CD's.

is a perfectly fine phrase, and does imply the purpose of needing said "something". Here, you can also substitute to because there is an associated aim/intention:

I need something to store CD's.

In the second example, however:

The PC is still the most popular tool for developing software systems.

a substitution with to develop doesn't work because the PC is not the agent which develops software systems, but the agent used to develop software systems. As such, an appropriate rephrasing would be:

The PC is still the most popular tool used to develop software systems.

And because this remains a statement of purpose, you can also say:

The PC is still the most popular tool used for developing software systems.

I should note that I now rescind my recommendation of the for the purpose of substitution test that I initially suggested in comments on the original post; although that was what I immediately thought of, it is in no way a comprehensive test nor definitive. For instance, it fails in the "for being" example and the "for visiting" examples.