Please help.

Thank you guys for stopping by to view this post.
My english is still bad today because of my laziness to practise and learn new vocabs everyday. So please someone help me with this.
1. Which one is the suitable one for this sentence?
I’m (at or in or on) your side.
Btw how I can distinguish the use of at, in, and on easily? I had asked some of our friends here about it. But it seemed that I were too stupid so I can’t get the explanation right.
2. “She’s sitting around with me and my father talking about her trip to paris.”
Can we use talk to replace talking?
3. “I’ll be paying it tomorrow.”
“I’ll pay it tomorrow.”
what’s the different of these two sentences in meaning?
Many thanks for your great explanation friends.

1. I'm at your side means you are right beside a person and supporting them in (usually) at time or trouble. I'm on your side means that I have taken a side in a fight or an argument. 

 2. Talking is current, it's happening now. She is talking. Talk is a noun or an action that happened and may still happen. What do you talk about? Did you have a talk with her? When we meet, we usually talk about sex. What was the talk about today? 

3. Both work. They are different responses but, for the purpose of this question, they are the same. When will you be paying the bill? Both responses are ok for this question.

Mystery, thanks for the explanation

Thanks mystery for replying.
1. So in what meaning I can used at school, on school and in school? I still couldn’t grab the concept.
2. Shouldn’t we add any “to be” before talking (verb + ing) as a verb?
3. So they two don’t have any differences in meaning at all?

1. You are on something. On the school, on the bus, on the deck, on top. You are at a place. At school, at home, at the theater. You are in something. In debt, in shock, in the water. What are the rules? There probably aren't any. You have to talk to people, read and try it out for yourself. Studying rules won't help much. There are exceptions for every rule. 

2. To be talking? It's great to be talking to you now. They were talking. On the news they were talking about Iran. I'm not sure what you mean by adding to be before talking. Is that rule? If it is, read number 1.

3. There really isn't  a difference between the two sentences.  Usually, the response mimics the question: "When will you be paying the bill?" "I will be paying it tomorrow."

"When will you pay the bill?" "I will pay it tomorrow."

2. I mean in present continues which uses verb + ing, the pattern is: subject + to be + verb ing + object.
So the sentence would be like this : ... me and my father are talking about her trip …

If you are talking about it now, yes. Right now, not any part of the past or the future.

We were talking about her trip. We will be talking about her trip. 

I guess I'm not getting the meaning of your question. Anyone else?

I hope this time I can grab the pattern well. Thank you so much Mystery.