Do I have to cite sources for my opinion when it was formed by reading other people's opinions? (undergraduate)
I would reexamine what you mean by &quot;agree with my opinion.&quot; Consider for example:

Einstein proposed the Theory of Relativity, which has been widely accepted by the scientific community and has withstood a battery of experimental tests over the last century [cite]. I agree that this theory is correct.

This last sentence is a little bit ridiculous -- your opinion is altogether irrelevant (unless you are an esteemed physicist). The is true in other fields -- saying you, as a student (or even me, as a professional in a different field), agree or disagree with Mark Twain or Charles Darwin or Thomas Aquinas is almost comical.
On the other hand, facts are facts regardless of who says them, and you are entitled to an &quot;academic opinion&quot; even now. So consider, instead, this much stronger paragraph:

Newton's laws of kinematics were widely accepted for hundreds of years, and have been experimentally verified many times [cite, cite, cite]. However, Einstein's recent work argues that Newtonian kinematics does not apply at very high speeds [cite]. Recent work with cathode rays [cite] seems to support Einstein's findings.

So in this case, you are clearly arguing that Einstein is correct and Newton is wrong (i.e., &quot;your opinion&quot; is that you &quot;agree&quot; with Einstein), but you do it with facts and logical reasoning, without ever mentioning &quot;agreement&quot; or &quot;opinions.&quot;
Now you had asked specifically about making a choice -- what to do when experts disagree. Even in these cases, you can present your analysis without ever mentioning &quot;agreement&quot; or &quot;opinions.&quot; Consider:

&quot;Jones argues A [cite] while Smith argues B [cite]. While there is no clear answer, Adams makes a compelling case for B [cite], noting that _. Indeed, in light of Adams' arguments (particularly his ontological argument), Jones' position seems wholly untenable.

In the above, it is clear that you &quot;agree&quot; with Adams and Smith, and your claim that Adams' case is &quot;compelling&quot; is very much an opinion (one that you will need to defend in the paper). But you never use the words &quot;opinion&quot; or &quot;agree,&quot; and the citations take care of themselves.
Nor are you limited to discussing others' ideas; you can also raise your own ideas:

&quot;Jones argues A [cite] while Smith argues B [cite]. While there is no clear answer, the only verifiable evidence is the exponentially rising CO2 levels. Recent results [cite] have only strengthened this body of evidence. Given the extraordinary success of this prediction, Smith's position seems superior to that of Jones.

In the second and third sentences above, you are presenting your own, novel ideas or analysis, and there is no need to cite anyone. Again, you are very clearly agreeing with Smith, and you very clearly have an opinion, but you never use those words, and the citations take care of themselves.