I assume you mean electronically? Like your computer goes phut or you lose all your email?
You should take steps to back up your files ie make copies of everything important from time to time. You can do this on disks or onto a pen drive. Simply select, copy and paste the important files / folders. The trick is then to put the disk or pendrive somewhere safe. properly safe, not on the bookcase.
You can also pay firms lots of money to store your data (banks do this) or you can use a free file storage website, lots available if you search online. However bear in mind that free storage may have certain limitations, and you need to be certain about who you are dealing with.
If you are talking about your birth certificate, premium bonds etc - a fireproof metal box will keep them safe apart from in very hot temperatures. A good old fashioned airtight biscuit tin is also good. Especially for biscuits.