If you understand copyright you can start looking for content. There are various sources of content out there, some will cost you money, some will cost you time, others are free. The type of content you're after and where you can get it will both depend on the nature and topic of your site.
The number one source of content is usually yourself, this is why it is recommended for people to make a site about a topic they experienced with. You undoubtedly also have friends interested in the same topic and they may be convinced to write for your site as well. Typically when someone else is writing for your site you provide them with an article signature with is usually a couple lines of HTML that explains their credentials and provides a link to their website (if any). If you choose to pay writers for articles you will often receive a higher quality of article, however your expenses will obviously be higher. Many people will write for free though, especially if they have a website. The reason is that writing articles for other sites is a good method of promotion, you will get the word out about your site and the link in your signature will not only provide direct traffic but also increase your link popularity, more on that later.
In some cases other sites may syndicate or redistribute their content, often they will do this for a monetary fee but sometimes they do it for free in exchange for a link or other recognition. You need to be careful when accepting syndicated content like this. If the content is largely branded by the other site then having it on your site can make your site look unprofessional and it can send your traffic away. In contrast if you only need to insert a link and you can reformat the content as you like then using such content can be very beneficial, especially when you're first starting out and need to build up content of your own.
Copyright law does provide for you the right to take some content from other sites. Depending on what type of site you run other sites may be your main source of content. For example if you run a resource site you could get all of your links from other resource sites which would safe you the time of tracking the links down yourself. What you couldn't do is copy the formatting or text associated with the links, that would still be copyrighted. It is imperative that you understand copyright law so that you know what you can and cannot republish. If you're ever in doubt, don't do it. A competitor's site may also be able to lead you to new content. For instance you may find a news story about a hot new technology on a competitor's site, this might lead you to writing your own story on this same new technology. For this reason it is important that you regularly monitor your competitor's site.