Here are the steps to encrypt your drive with BitLocker, which is the tool provided by Microsoft on Professional Windows versions.
1.Sign in to Windows with an administrator account.
2.Go to Start, enter encryption, and select Change device encryption settings from the list of results.
3.Select Manage BitLocker, select Turn on BitLocker, and then follow the instructions.
If you have a Home addition you can upgrade it to Pro for around $100 by going to Settings > Update & security > Activation > Go to Store. There are other options for encryption, if you don’t want to pay for the upgrade. Take a look at TrueCrypt which isn’t as secure as BitLocker but should still work ok.