Just as the title says, back up your WordPress. Doing it just once is a start but if your regulary adding content then it will need to be more often. There are many plugin tools that can do it and there is also an export tool in your WordPress dashboard that can export your content.
Also check out your hosting company as many offer a back up services and can be free. So find which way suits you best, find a way to remind yourself to do it often and keep it all safe.
How To – personally I would first check out the hosting company. If they can do it for free and do it regularly then its something you don’t have to remember.
If your hosting company don’t offer this service then in your WordPress dashboard check out the Tools section. There is an export tab which will export your posts and pages to your own computer.
Also check out the plugin section and search for back up. There are lots to choose from depending on how much you want backed up and your own level of nerdy knowledge.