I'm making a book and want to illustrate it too. So do I have to copyright them seperatly or can I copyright them all together? And if I make them into a series do I need to copyright each book separately?
I don't know about the US, but in the UK copyright is assumed. If you wrote it, you own the copyright on it. To make it clearer you put a copyright notice on your work, but this isn't necessary. Intellectual property is protected by law, (even if this doesn't happen in practice).
One exception to this is if you are working when you produce the material. In that case, your employer owns the copyright.
I don't know about illustrations, but I assume the same applies. You created it so it is your work.
Because you are the sole owner of all the rights in the work, you should be able to copyright it all together. It sounds like you would be eligible for the Single Application copyright form. Take a look at Circular 11 from copyright.gov. https://www.copyright.gov/circs/circ11.pdf There is very specific language on page 2 that directly speaks to your situation.
The series thing is its own can of worms, but since you aren't sure whether you want to do that right now, you should copyright this book on its own.
Some additional info if you want it:
Copyright is indeed assumed in the USA. You cannot copyright an idea or concept, but the moment you put your expression of that idea into a fixed, tangible form (a book, a movie, a recording) copyright attaches. In other words, JK Rowling can't sue you if you write a story about a "wizard school" as long as you aren't trying to call it Hogwarts.
It is worthwhile to register your work with the copyright office. Registration is basically a paper trail, and it's useful to have for anything you want to put out into the world. It announces that you had this specific expression of an idea and put it in fixed form before anyone else. If anyone ever tries to pass your book off as theirs, you have a relatively simple, clear-cut answer to their claims and may save yourself a lot of time and expense in court. Without registration you won't be able to access handy things like "statutory damages"; you'll have to prove, painstakingly, how much money the thief actually cost you, and that is difficult to prove. The "poor man's copyright" (envelope trick) is pretty much asking to remain poor if anyone steals your work.
You can do the initial copyright registration yourself. When it's time to review publishing contracts, however, I strongly encourage you to hire a lawyer experienced in this field to review the terms, negotiate future rights, trademark considerations, etc. I wish you all possible success!
Disclaimer: Not a practicing lawyer, and not offering legal advice; just giving you my best info based on my experience with this subject, which is: Have a JD, have worked in this field, and have successfully submitted copyright and trademark applications with the gov.