Assuming your MBP is running OS X Lion or Mountain Lion:
Reboot the machine and as soon as you hear the POST chime (Bong!) hold down the Option key until you see a choice of boot options on the screen.
Choose the Recovery Partition to boot from by selecting it and pressing enter. Once the machine has booted to Recovery, select Disk Utility. From Disk Utility, select to erase the hard drive and format it as Mac OS X Extended Journaled. Make sure the GUID scheme is selected (it should be by default) and press apply.
Once that finishes, go back to the main Recovery menu and select to install Lion (actually I recommend Mountain Lion). Note - that you will have to pay for either a copy of Lion ($30) or Mountain Lion ($19.99). You will need your Apple ID and Password. If you don't have one, make sure you get one before starting all this.
If you need further help, post back and someone will jump in and assist.