Because the rights afforded by copyright law are exclusive to the copyright owner(s), you will infringe copyright if you do any of these things without the permission of the copyright owner(s) - for example, if you copy or adapt a copyrighted work, or make it available on the Internet.
The best way to avoid copyright infringement is to ensure that you don't use anything created by anyone else.
If you do use someone else's work, make sure you have the necessary permissions – this will usually take the form of a license from the copyright owner(s), which you may have to pay for. There are certain instances where you may be able to use excerpts of copyrighted material without a license – for example, if you use a small part of someone else's work for the purpose of criticism or review, or if your use constitutes "fair use" under applicable law (particularly U.S. law) – however, discussion of these exceptions is beyond the scope of this guide. If you intend to use any part of a copyrighted work in reliance on any of the statutory exceptions, you should seek legal advice first.