https://www.microsoft.com/en-ca/software-download/windows7
Download Windows 7 installer to a USB drive (8GB or greater formatted in FAT32 with nothing else on it)
Download Belliarc utilities.
Run Belliarc utilities from your current hard drive while in Windows that is installed on it to extract your Windows 7 key. Write it down. Double check it.
Next, restart your computer with the USB stick you just put the Windows installer on plugged into a USB 2.0 port on either the front or back of the computer.
When you restart you will see a message that says something like "press F2/DEL for BIOS, F12 for boot menu" (or you might not see that at all). In any case, spam those 3 keys (press them repeatedly as the machine starts up)
From either the BIOS or the boot menu select the USB flash drive as the primary boot device and restart again. You will see a message that says press any KEY to load from (your USB drive). Press any key. I like the space bar.
Follow the instructions on screen. Enter your windows 7 key when prompted to do so.
NOTE: there are multiple versions of Win 7 with Home Premium being the most common. Take note of which version you have and make sure to install that version. Make sure you run the same bit version, either 32 or 64, depending on what you have now.
It's actually all much simpler than it sounds.
By the way if you have never installed Windows before you have also never installed drivers. I'm going to guess you have an OEM (brand name) machine in which case all you have to do is go to the support section of their website and put in your model name or service tag and you can download the drivers. Just install them one at a time the same as any program. It's very simple.
If you have a machine made of consumer parts, take note of those parts and download the appropriate drivers from the various manufacturer websites. You do not need to visit the manufacturer website of your motherboard for it's chipset driver, as INTEL or AMD offer those on their site, but you will need the motherboard manufacturer site for addon-chips such as sound, LAN, additional SATA controllers, etc etc.
You will need drivers for
Motherboard
Video Card (Go to NVIDIA or AMD for this, not MSI/ASUS etc)
Sound card (if you have one)
Wireless card (if you have one)