Do you have a rough idea of how many GB's you have?
Go through your Docs, Desktop, Pics, Vids, and Music folders and tally those up.
They are the most important.
Things like emails, downloads usually are either stored in the cloud or can just be re-downloaded.
If you only have a few GB's to save, get a couple of USB sticks and rotate their usage.
For example, get three sticks, and every week or month save your data to Stick1, then next week/month, save to Stick2, etc.
But probably the best option is getting an external USB drive to copy your data to.
Regularly copy your data to it and then keep it somewhere safe; another room, in the car, next door neighbour, your Mum's place - things like that.
Where you store the backup device, and how often you do it, all depends on the importance you place on the files and what you are trying to guard against.
If your main worry is deleting a file accidentally, save it to a USB stick and keep it in your desk draw.
If you live in a high theft area and they are targeting PC's, keep the backup in another room.
If bush fires are a concern, keep the external backup drive in another location.
You get the point...