Before Installing Windows 10 Technical Preview please make note that Windows 10 Preview is only for the users with x86 processor on their PC. If you are not sure about the version of your Windows PC Follow below steps.
Right Click On My Computer>>Click On Properties
Now if you are sure that your PC is running on x86 processor check for some more information on your device.
1GHz or faster with support for PAE, NX, and SSE2 Processor
1GB (32-bit) or 2GB (64-bit) RAM
16GB Hard disk space
Microsoft DirectX 9 graphics device with WDDM drive Graphics card
Microsoft account and Internet access
To Get Access for Windows Store or use Windows Apps 1024x768 monitor resolution.
Getting Technical Preview
Visit :http://windows.microsoft.com/en-us/windows/preview read through the information.
Click the Get Started button, Log in to you Microsoft account to join the Windows Insider Program.
Follow the steps to go to the download page. Download the appropriate ISO file to your hard disk.
Download appropriate version for your computer. Its free of cost you just need to download it.
Now Install it as you normally install any software.
Creating a virtual machine
After Installation completes successfully you will able to see VirtualBox Manager screen.
Click on New Button
You will be ask to create hard drive.
Allocate name and disk space as per your choice but not less than 1GB
Installing Windows 10 Preview on Virtual Box
VirtualBox can install Windows 10 right from the ISO so just double click on storage
Select IDE controller, and then click the Add CD/DVD drive button.
On Popup click on Choose disk button
Once you select the ISO, VirtualBox access ISO and begin the Windows 10 installation
Choose Language and Click next
Your system will reboot while installation.
On start up you will get screen for settings.
You will be than prompt to login to Microsoft account.
In notification select One Drive.
Now you will not have to do anything and installation will continue till end.
Restart your computer and test the Windows 10 Preview.