If you have Windows 8, 8.1 installed in your computer then you will see a windows icon in your taskbar near the time and date area.
Once you get it just click on it and it will have this pop-up window:
Once you click next it will ask you to enter an email and after clicking next you have reserved Windows 10 for yourself!
By doing this it will “guarantee” that you can get the update with a priority when the servers are being hammerd during the initial release. This way you can be one of the first people to get Windows 10!