I'm looking to trademark my own work and I want to know how. Everything I found online is exclusively about trademarks for COMPANIES and PRODUCTS, but not intellectual property like a manga series or a TV series or anything like that.
You don't need to register for copyright; works are protected automatically. (Do note that "[p]rotection does not, however, extend to the title[*] or general theme [...], the general idea or name for characters depicted, or their intangible attributes.") You may still wish to register for copyright anyway though:
Why should I register my work if copyright protection is automatic?
Registration is recommended for a number of reasons. Many choose to register their works because they wish to have the facts of their copyright on the public record and have a certificate of registration. Registered works may be eligible for statutory damages and attorney's fees in successful litigation. Finally, if registration occurs within five years of publication, it is considered prima facie evidence in a court of law. See Circular 1, Copyright Basics, section "Copyright Registration" and Circular 38b, Highlights of Copyright Amendments Contained in the Uruguay Round Agreements Act (URAA), on non-U.S. works.
To register a copyright with the US Copyright Office, you can either do the paperwork online or mail it in. Either way, there will be a fee, but it's significantly less if you do it online.
You will need to submit one or two copies of your work with the application. If the work is unpublished, you only need to submit one copy, otherwise see the section titled "Deposit Requirements" here. If you're doing it online, see this list for a list of accepted file types.
You should be able to just create an account here and follow the instructions if you decide to do it online.
[*] Titles cannot be copyrighted. Titles for a series (not individual books) can be trademarked. See the guidance here. I'm not going to go into the specifics of getting a trademark, but I will mention that you can hire a trademark attorney to help you through the process. They advise you to do so because the success of your registration depends on what similar existing trademarks are out there, among other complicated factors. I'll also quote something relevant from the USPO:
In the United States, parties are not required to register their marks to obtain protectable rights. You can establish “common law” rights in a mark based solely on use of the mark in commerce, without a registration.