Today is warm, so we will go outside.
Which of the following word order is correct?
(a) Heute ist es warm, so gehen wir raus.
(b) Heute ist es warm, so wir gehen raus.
As already pointed out, "also" is the correct word to use here. In this case, since "also" is a conjunctional adverb (I don't actually know, whether it would be called this way in English, the correct German term is "Konjunktionaladverb"), the correct word order is "also - verb - subject". This applies also to all other words of similar type like "dennoch", "doch", "ebenso" or "insofern" (and many more).
But let me add some other uses of "so" that were not mentioned already. You can use the German "so" in the sense of the English "if" - in very formal circumstances:
If God wants, it'll be alright.
Another usage that you might frequently encounter is in the sense of "like" as a filling word in colloquial language:
He said "Get lost" and I answered "No" (feel free to add "like": He said like, "Get lost"...)
In German I don't believe you can use so as a conjunction, but rather also, which fills the 1st position in the sentence or clause sending the subject to the 3rd position.
You only use so when you comparing people or things:
Ich bin so alt wie du
I am as old as you (are)
You are taller than he (is)
It can also mean
Dont look at me like that!
That's the life.
In this context, so is not used here but also instead:
Heute ist es warm,