If you do not have a recent disk-based backup, your computer is like a ticking Time bomb. You are only one disk failure away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail.
If you are using another direct-to-disk backup method that you prefer, and you currently have a recent backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast. (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).
Attach your external drive and use
System preferences > Time machine ...
... to turn on Time Machine and specify what drive to store your Backups on. It may ask to initialize the new drive, and that is as expected.
Time Machine may spend all afternoon making your first full backup. You can continue to do your regular work while it does this. The first Full Backup is by far the biggest backup. After that, it will work quietly and automatically in the background, without interrupting your regular work, and only save the incremental changes.
Time Machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.
How to use Time Machine to Backup or Restore your Mac:
https://support.apple.com/en-us/ht201250
