Despite your best efforts, something bad can happen to your computer. Hardware or software failures in addition to security threats can make your data disappear. The best protection against such events is an adequate backup strategy.
For many people, occasionally copying important files to a flash drive, CD/DVD, or external hard drive is sufficient for backing up their data. If you use a flash drive or external hard drive for backups, make sure it is not always connected to your computer, otherwise whatever bad thing happens to your computer might destroy your backup as well.
If you have a substantial amount of data you need to backup, or regularly update or create lots of files, you may be best off with a software backup solution. Backup software usually compresses the data so it takes up less space and makes it easy to copy the data to multiple CDs or DVDs. Most operating systems come with a simplistic backup software program, just search for "backup" in your operating system's help menu. Linux users will wish to look at their software repository for backup software. Otherwise a significant number of low-cost software programs and cloud services are available.
Consider moving a copy of your backups to a safe location, such as a fire safe or safe-deposit box. If you are concerned about the security of your backups, many of the software solutions include encryption options to protect the backup from being viewed by others. Several companies offer cloud backup storage that you can utilize for a reasonable cost. Cloud services can provide you secure off-site storage without much effort on your part.