What is the size of your current internal hard drive?
How much of that space is currently used?
Do you already have an external hard drive?
If so, what capacity?
Do you have a USB flash drive?
If you haven't yet bought one, I suggest you get a USB3 flashdrive with at least 64gb of space, 128gb would be better.
The problem with Time Machine is that it will quickly consume all your available backup space, and what's worse, the backups that it creates ARE NOT BOOTABLE (shouting intentional). This could become very frustrating in a "moment of extreme need" -- such as if you have an "I can't boot!" situation.
Cloning apps like CarbonCopyCloner and SuperDuper will create true bootable cloned backups that will be (for all intents and purposes) an exact, bootable copy of your internal drive. If you have an "I can't boot!" experience, just connect the clone backup drive and boot from that instead. Takes about 3 minutes, and you'll be up-and-running again with everything "right there in front of you".
At this point you can begin working on the internal drive to get it fixed.
If for some reason the drive can't be fixed (hardware failure), just put a new drive in, and RE-clone your cloned backup BACK TO the replacement drive, and you'll be where you want to be.
CCC will even clone your recovery partition, and it can also "archive" old backup files, so that you don't lose previous versions of important files.
My opinion only, but for most backup situations CCC or SuperDuper are SUPERIOR to Apple's Time Machine. No comparison...