What is the correct way of saying the following?
The decision has been made in my side.
We request the people that the decision be made on our side.
"In one's favor" is the existing idiom to communicate what it is you're trying to say. That would make your sentence:
"The decision has been made in my favor."
-or-
"We request that the decision be made in our favor."
For "the decision has been made in my side" I would simply say: "I decided" or "we decided" - if it is not only you but a group of people. For the second quote: "We request the people that the decision be made on our side." I would say something like: "We request the people to let us decide." or "We request the people to let us make the decision."