So is primarily used in writing as a coordinating conjunction, whereas the phrase "so that" is generally used as a subordinating conjunction.
A coordinating conjunction, according to englishclub.com:
joins parts of a sentence (for example words or independent clauses) that are grammatically equal or similar. A coordinating conjunction shows that the elements it joins are similar in importance and structure
englishclub lists seven coordinating conjunctions:
and, but, or, nor, for, yet, so
Examples of a coordinating conjunction using so:
"She is kind so she helps people."
"I want to work as an interpreter in the future, so I am studying Russian at university."
In these cases, both parts of the sentence which are joined are grammatically equal or similar in importance and structure, and one is not necessarily dependent on the other one.
whereas a subordinating conjunction:
joins a subordinate (dependent) clause to a main (independent) clause
Here are some common subordinating conjunctions as well:
as long as,
in order that,
In short, use so primarily as a coordinating conjunction, and the phrase so that as a subordinate conjunction and you'll be fine.
Some helpful sites just for good measure: