If your computer is still using a big Hard Disk Drive (HDD), it is highly recommended to replace your HDD with a Solid State Drive (SSD) as your primary hard drive. The main difference between an SSD and HDD is an SSD is flash storage and has no moving parts within it. Because of that, they're smaller and will take up less space in a PC. An HDD is made of magnetic tape and has moving mechanical parts within it and are more prone to being damaged than an SSD. You can pick up a 500GB SSD for between $60 - $80 on Amazon, depending on which brand you go with. Leo recommends this one for example.
Of course, you don't have to completely get rid of your HDD when you get an SSD for your PC. You can continue to use your HDD alongside your SSD at the same time: using your SSD to primarily boot your computer and applications from it, and use your HDD to store documents and big files on it.