|If you are going to get a new hard drive, then if you have space in your PC you can keep your old hard drive as a backup until you have got everything up and running on the new hard drive.|
Secondly as you have not said whether this is a laptop or desktop.
Regardless you can get a USB caddy so that you can use the old hard drive as an external hard drive. Again you can leave your data until you have got everything up and running on the new hard drive.
Backing up data requires you to consider what data you want to back up.
Examples of data to back up are (note this is not an exhaustive list):
1. Your emails
2. Your bookmarks (favourites)
3. Your personal downloads (e.g. music, videos, games)
4. Any installables you downloaded that you want to keep
5. Your Office data (e.g. word documents, excel, powerpoint and other such documents)
6. Your personal photos and pictures
There are third party back up tools that claim to backup all your data you could look at. I have not used those myself as I do mine manually.
So long as you know your important data you should be able to identify it and identify the storage space required to accommodate the data and choose a suitable back up medium for storing your precious data.
Also as a fail safe mechanism you should have at least two different copies of backups on two different media just in case.
☺ When everything else fails, read the instructions.