Copyright exists in anything you create, as soon as you write something down or record it in some manner, you have copyright. The work must be your own, so plagiarised material is excluded. If you are employed or have a contract which stipulates that copyright passes to whoever has commissioned your work, then copyright vests with your employer.
In the UK and European Union, copyright lasts for 70 years after the death of the writer, or if the work is published after death of the writer, for 70 years after publication.
A general rule is not to surrender copyright unless you have negotiated suitable compensation. Retaining copyright gives you the right to be rewarded for your work and if successful, may continue to benefit your family for decades after your death. Look at the estate of JRR Tolkien, the author of “Lord of the Rings” and consider the royalties paid the owners of the copyright from the movies that have been made.
You don’t have to actually assert copyright on your work but adopting a “belt and braces” approach does no harm and will help to deter those who may seek to infringe your rights. I tend to follow this myself whenever copyright is not passed on with my work, and the usual form is (c) My Name 200X which is the year it is published or created if not published.
There are exceptions to what is subject to copyright. For instance, there is no copyright in a title so you are free to use someone else’s title for your own work. You need to take care as you may fall foul of what is known as “the tort of passing off”; in other words, you can be sued if you are attempting to benefit from naming your own work after someone else’s popular title in an attempt to make money by trading off the title.
There is also no copyright in ideas unless you have documented in fine detail what that idea is and the idea is 100% genuinely original.
If you use photographs in your work, then copyright will vest in whoever took them, even if you have commissioned them yourself. If you want copyright to vest in you, you must ensurethat the commissioning agreement stipulates this.