Have you read this guide:
Shared folders

The process of creating a shared folder is well explainded in this guide.
In short:
1) In your VM properties set a shared folder on host machine.
2) Boot the guest machine and run the following command:

net use x: vboxsvrshare

where x - is the name of the drive with virtual folded. You may leave it as is, and share - is the name of the folder you've created on host machine.