The basic 4 system requirement for Windows 10 are here as following :-Latest OS
If you have planned to upgrade from exsisting Windows version then you will need Windows 7 with service pack 1 and Windows 8.1 update. You must have installed all updates before tying to upgrade.
In case if you have downloaded Windows 10 ISO then you can do a clean install.
1 Gigahertz or faster, but I would recommend atleast 1.6 or 2.2 ghz processor.
Atleast 20 gb for Windows installation and running programs.
Any graphics that supports Direct X 9.0