For formatting and installing Win7,you'd need a Bootable Windows 7 DVD,or a bootable USB loaded with Win7. Then,you have to put the DVD in the drive,if your choice be the DVD,or connect the USB to the port if your choice be USB.
Then restart your system and you have to get into your BIOS and goto Boot Options,and set the First Boot Device to either DVD Drive or the USB,whichever you're going with. And if it's DVD,you'll be asked to press any key to boot,after restarting the system,saving the BIOS settings,for which you need to press any key..If it is USB,it'll start the installation automatically. Once the installation starts,a window will appear with options to select the Language,Time and Currency Format,and Keyboard Input method.Select all those according to your needs,and click Next. Then a window will appear with an 'Install Now' button. You have to click on that,and it will take you to a screen where you have to select your OS type- X64 or X86,which you need to select according to your system specs. Then the next is License agreement screen,where you need to select '....I accept...' and click Next. Then,you maybe asked to enter the Product Key. Do the same and click next. On the next screen,you'll see two options- Upgrade and Custom. Click the Custom. You'll be taken to a window where all your HDD partitions are listed. Look for the one with the current OS loaded. Select that and at the bottom of that window,you'll see something which says Drive Options. Click that and click Format.It will ask for confirmation and you need to press Yes.Then once the format is complete,click Next. The installation will start. Then everything else is automatically done,and in between,it will restart once.( N.B: If you do the installation with USB,once the system restarts at this point,get into your BIOS again,and make you HARD DISK the FIRST BOOT DEVICE,or you'll go into the beginning of installation once again.) After that,the installation will continue,and once everything finishes,it will restart again,and then a screen will appear asking you to Personalise your settings and create your User account. Do that everything according to your needs,and then it's done.
After that,you'll need to install all the drivers and softwares. Be sure to have a backup of all the softwares you currently have,and backup any important data you have in the Partition with current OS,before formatting.