>As I have never been to this debates or stuff, I have to ask what is a source and what is not considered a source?
A source is anything from which you learned some information. Anyone and anything can be a source, a man on the subway, the back of a cereal box, graffiti in a toilet, and so on. What is important is determining which sources are reliable and which aren't, in other words, which information can you trust and which information can you discard.
>Is Wikipedia a source? Is a newspaper a source? Is Buzzfeed a source? Is Marxists.org a source?
Yes to all.
>How do people of all political opinions agree on what is considered a trustworthy source or not?
There are many ways you can determine whether a source is trustworthy, by comparing it to other sources, by analysing the methodology of the source, by using critical thinking, even by looking into the author's motives, etc. For example, historical events have to be pieced together from various contemporary historians. The more contemporary historians write about an event, the more likely it is to have occurred. There's debate about whether Jesus Christ existed as a historical figure, and if he did, what was he like. There are only one or two contemporary historians that mention someone who could be Jesus Christ, but then there are many others from that area and time period who don't, yet we'd expect that if miracles and extraordinary things happened, those who lived at the time would write about it. So some people believe Jesus existed because of the existing historical evidence, and others don't because of a lack of historical evidence.
>On the internet you can find any webpage agreeing with you on whatever insane thing.
Another way, which isn't always reliable, but reliable enough when compared to "any webpage", is to look at the author's expertise. You'd trust a historian writing about history before you'd trust some random dude with a ifuckinglovehistory.blogspot.com webpage. Looking into the author, their previous works and (when talking about politics, economics or sociology/psychology) their political leanings, can also help you decide one source over another.
>So like how do you decide what can be considered a source in good faith????
The number one way though is to read both, and use critical thinking skills to make your mind up. But that requires a certain level of education and the ability to critically think, which can only be learned through practice, that is, reading reading reading.
Here's good practice for you: read Charles Murray's "The Bell Curve" (https://en.wikipedia.org/wiki/The_Bell_Curve)
and then read Stephen Jay Gould's "The Mismeasure of Man" (https://en.wikipedia.org/wiki/The_Mismeasure_of_Man).
The former is the famous thesis that human intelligence (IQ) is distributed in a population along a bell curve, while the latter is an answer to that book, showing that IQ isn't a thing that exists, but a constructed measure of how well you can do the examiners' tests. Stephen Gould answers to the claims in the Bell Curve by explaining his opponent's position, then arguing against it by citing conflicting studies, attacking the argumentation, the reasoning and premises, and so on.