SO (c)
 
      I didn't know what to do, so I decided to do nothing
 
THEREFORE (fc)
 
      England is a cold country. Therefore, palm trees cannot grow there
 
 
"So" is extremely common when speaking and also the most used result connector when writing.
"therefore" is only used when writing and it’s quite formal and very elegant (but you can’t use it many times in the same text or it would be too much)

NOTE: Reason and result are interconnected:
 
A B (A is the cause/reason of B)
B A (B is the result of A)
 
      I speak English because I am an Australian   (reason)
      As I am Australian, I speak English               (reason)
      I am Australian, so I speak English               (result)

abbreviations:  (c.)= conjunction   (f.c.)= free connector    (see the important difference here: Kinds of Connectors)

For Spanish Speakers

SO (c) = así que
THEREFORE (fc) = por (lo) tanto