Choosing and Using a Theory

Which theory do you choose?
A lot about being a scientist is learning new skills. However, you also need to develop your knowledge of the problem.
You need to read about theories.
You need to think about the problem you're trying to solve.
As you find out about theories, you need to see if they're relevant to your problem, and to evaluate if they're right.
(You need to discuss these theories with your supervisory team, colleagues, and anyone you can find.)

So what do you use a theory for?
I think most importantly, it supports your thinking.
It can also support your method.
It's also good for citations and arguments.
One key is to reuse what other researchers have already done, but to add your own thing.