If you're planning to move to Windows 10, actually installing the OS is the first area you could potentially run into problems with. Installing a new operating system requires a certain amount of free space on your drive so that it can be downloaded and certain elements can be run successfully.
For Windows 10, the space requirement is 16GB, which should be kept free on the main system drive the computer uses. This is actually the same as previous versions of Windows, so if you've upgraded before you can most likely do it again.
To check how much space you have left, go to My Computer (or This PC depending on the version of Windows you're running) where any drives you have will be listed. You can see the remaining space indicated underneath each drive, or you can right click and select Properties for a clearer overview (the system drive is usually C:).