You need to install these things :'
1. The drivers for all the hardware and parts you have put it it. You can get them from the manufacturer's website.
2. An Antivirus, get a free Avira or Avast Antivirus, they would work just fine.
You do not need a Paid Antivirus, it is a waste of money as most tech sites concluded.
3. You might need some free apps like
VLC Media Player, Libreoffice (free alternative to Microsoft Office), Gimp(free alternative to Photoshop), Google Chrome, Utorrent for torrent downloads, Sumatra PDF (For Viewing PDF files), Thunderbird (For Emails), Adobe Flash (for online flash content like Youtube), 7zip (for viewing and creating Zip and RAR files),
4. You need some other solutions to keep your Windows clean. You should have cCleaner, it is a good tool for keeping your system clean from junk.
I hope this helps in getting a rough idea of what you should install or not. I hope this helps. If you have any other queries then feel free to ask them below. I would be happy to help.