Well, I'll try to find the article. I would also advise you to check the link above ("meet the verb").

For now, just a few words about the origine of this verb.

OK, собирать (sobirát’) impf. — собрать (sobrát’) pf.
to gather, collect
- Yes, that's right. It's the primary meaning of the transitive verb (собирать что? to gather, collect what?).
Some examples that first come to my mind:
Он собирает почтовые марки. - He collects post stamps. i.e. He makes a collection of them, I think you get it.
Я собираю вещи в дорогу. - I'm collecting (packing) things for a journey. (literally: "to road").

собираться (imperfective) / собраться (perfective)
is a reflexive verb (note the -ся).
And being a reflexive verb, it is intransitive (i.e. it cannot take a direct object).

It either expresses the "passive voice" (to be being collected, gathered) - not so frequently used in this sense;
or it expresses the reflexive meaning ("to get collected, gathered" or "to get prepared"):
- На площади собралось много людей.
- A lot of people "have been collected" in the square (i.e. They have been met there, e.g. to have a mass meeting).

E.g., the direct meaning of "собираться" can be seen in the following example:
- Что ты сейчас делаешь?
- Я собираюсь в дорогу.
- What are you doing now?
- I'm getting ready for a journey. (Lit.: I'm "getting myself packed" for a journey) - this meaning is still related to "packing things", "collecting things", can you see it?

Now you are close to understanding its meaning in sense of "getting ready" for something:
- Я собираюсь отметить свой день рождения, поэтому я иду в магазин за продуктами.
- I'm "getting myself ready" to celebrate my birthday, that's why I'm going to a shop for some food.

But generally, "собираться" can just mean "to have an intention to do something", it does not necessarily imply you perform some actions. It can focus more on some kind of "mobilizing your internal resources" to do something
- Я собираюсь навестить своих друзей в следующем месяце.
Can be understood as "I have an intention to visit my friends next month", "I have a plan to do it", "I think I would probably do it if something does not prevent me" etc.

OK, I hope I'll find and translate the article a bit later. Я собираюсь найти и перевести эту статью чуть попозже (good example, yeah?)