An APA essay is simply an essay. APA specifies a lot of details, but most of them are similar to other essay styles: double-space, indented first line, no extra spaces between lines. The most important criterion for an APA essay is how to cite sources. APA essays use an author-date style of citations. Some other styles use variations on the author-date format (See Harvard style), but APA has a lot of specific details, most of which you will probably never need to know.
The main things you need to know for an APA essay (in addition to those cited above) are that APA requires a separate title page and a separate reference page. Everything that APA specifies can be skipped, so if you're in first-year, writing a 600-word essay, ask your instructor if you need a separate title page and a separate reference page (especially if you only have one or two references).
The key detail for an APA reference is the in-text citation. This is when you acknowledge where your information came from. Most citations should be from peer-reviewed journals. That means an expert wrote it and other experts criticized it before it was published. In the text, you provide the last name of the author and the date of the publication (Smith & Smith, 2020). Notice that the citation is in parentheses and the period ending the sentence comes after the closing parenthesis.
You can cite any kind of source in APA, but peer-reviewed sources are the most reliable, so you don't end up citing "Fake News." Do not cite a URL (http://...) as an in-text citation. That is never correct. For online sources, the author is either a human being, or the organization (e.g., CDC). Sometimes, you might find an online source such as The New York Times that is not the author but doesn't supply the author's name. In these cases, you can cite the title of the article. But dig deep; it's rare that you can't determine the author.
The other important detail for an APA essay is the reference list. The reference list comes at the end of the essay, and lists all the details about how to find the sources cited in in-text citations. Only sources cited in the paper should appear in the reference list; it is not a laundry list of related sources, nor a list of reading you might have done.
The reference list begins with a bold and centered title: References. Each reference is listed alphabetically. Each reference should contain four elements: Author, date, title, and publisher. Authors are listed with last name and first initials. The date is in parentheses. If the document is not part of a greater whole (i.e., it's not an article in a journal, or a chapter in a book) then the title is in italics. Otherwise, the name of the journal or the book is in italics. Most peer-reviewed journals have a digital object identifier (DOI) these days, so include the DOI, if you have it. The DOI should be formatted as a live link to the source (hyperlink).