Use sources and references to increase the credibility of your post, show where your information comes from, and attribute the work of other people when it has been included in your post.
If your post contains information that isn’t common knowledge, then people may distrust it. In cases like these, you should find a credible online source that agrees with the information you’re providing. For example, it isn’t well-known that Ground-type Pokémon can be paralysed; so if I were to mention this in one of my answers, then I should link evidence that the information is correct. This will make readers more confident in the answer.
If you found information from a particular website, then providing a direct link the source is the right thing to do. Link to the exact webpage instead of simply citing 'Bulbapedia', for example. This means that other people can see where the information comes from and judge for themselves whether it is trustworthy. It also makes sure that the people who made the content get credit (and maybe some kickback). If you don't link to your sources, then people may assume you're accountable for the information and ask you to explain it.
It is not necessary to provide a source for information that is well known by the community, or that is self-evident in the post. For example, if I can justify in my own words why Intimidate is a strong ability in competitive play, then I don’t necessarily need to reference a source that agrees with me (though that might make my argument stronger). Other times, you might gather information through your own investigation, e.g. by testing something in the actual game. If that's the case, you should give some indication of that, and explain what your process was.
If you include original work that isn’t your own in your post, then please attribute it to its source. Include a link to the page where you found the text, graphic, etc. If you don’t do this, you’re effectively plagiarising and we will remove your post. You can use blockquotes to indicate when a part of your post comes from an external source; a guide to this and other formatting tricks is available here. It’s smart to quote external sources in case they ever go offline, but please include your own writing in your post as well. (Note: ideas and strategies are not owned by anybody and aren't "original work", but it's nice to give credit to the people who thought of them.)
When researching and referencing posts, please be careful to use credible sources. Articles, databases, guides, etc. created and fact checked by multiple people are more trustworthy than random YouTube videos and forum posts. Take care not to mislead people -- if the information you’ve providing is dubious, then make a note of that (and consider posting it as a comment instead).
It is a good idea to put the link to your source near where you’ve placed the information or work that comes from it (or otherwise provide indication of which source matches with what information).