To show consequence:
* "It was raining heavily, so we decided to stay inside."
* "I studied hard for the test, so I'm confident I'll do well."
To emphasize a point:
* "I was so tired after the hike, I fell asleep immediately."
* "The food was so delicious, I had seconds."
To introduce a reason or explanation:
* "I wanted to go to the party, so I asked my parents if I could."
* "She couldn't make it to the meeting, so she sent her apologies."
To introduce a contrast or contradiction:
* "I love pizza, so I was disappointed when the restaurant ran out."
* "He's a great athlete, so he's surprisingly bad at video games."
To introduce a question or request:
* "So, what are your plans for the weekend?"
* "So, could you please pass the salt?"
In a phrase:
* "And so on"
* "Up so early"
* "So that..."
The context of your sentence will determine the best way to use "so."