So...
What is a good way I can say 'so' in a sentence. As in: I forgot my jacket SO I was cold. Or. I don't like lettuce SO I don't eat salads. I have heard "así que" and "entonces" and I'm sure it varies by region too.

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I wouldn't use "entonces" in these cases. "Así que" sounds perfect.

You'll usually want to use "entonces" as an equivalent for the English "then".

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Hm... We would probably say the whole sentence different in Spanish:

Estaba con frio porque me habia olvidado la campera (I was cold cause I had forgotten my jacket).

Still, as you say it in your example, I would translate -> Y por esto.

So can be translated in many different ways... it really depends on the context. Some examples:

por lo tanto
por lo cual
así
tan
tanto
de este modo
de esta manera	
por tanto
por consiguiente

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-"por lo que"
-"y es por eso que"

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Or "y por esa razón"

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I would use "así que".