Instructions
Create a new email address. Steam accounts are tied directly to your email account, and attempting to create a second account with an already registered email address will give an error. If you have a secondary email account, such as one used as a "spam" account on websites you don't wish to give your real account to, use that. Create a new account with one of the many free services available if you do not have one.
Open Steam to create a new account. You will need to create a new Steam username, which is a unique name that cannot be changed. Your first choice may be taken already, so keep pressing "Check Availability" until you find one that isn't already being used. Enter a password and the new email address you created previously, then create a security question and enter the Captcha image for a final security check. Accept the Steam Subscriber Agreement and your account will be created.
Start Steam if it isn't already started and log in to your new account.
Recreate your friend list by selecting the "Friend" menu at the top of the Steam window. Select "Add a Friend..." to add friends directly by entering their Steam names or email addresses. You can also search for friends by their nicknames -- names that can be changed unlike the Steam username -- through the "Community" menu. Click the "Community" tab and do a search for your friend's name to search for his profile. This also works for Steam Community Groups.
Submit a ticket to Valve about your stolen account if you haven't already done so. This can be done through Steam's Help menu.