If you have anything on the computer that you wish to keep then my first advice is to begin backing the computer up using Time Machine, operating ANY device (computer, phone, iPad etc.) without a backup plan is a 100% guarantee of lost data. If. you are not familiar with Time Machine please click Back up your Mac with Time Machine. Time Machine needs an external HD to backup to, I recommend the following https://eshop.macsales.com/item/OWC/ME3NH7T02/
To address your speed problems, as I initially mentioned you bought a base model iMac and the ONLY thing that will make it faster is to buy an external SSD and use that as your startup drive. Instructions for doing this are in How to setup an external SSD as your startup disk. The external SSD I recommend is the OWC Envoy Pro SX, this will speed it up dramatically. If you don't want to do the work yourself you can take the computer to your local independent Apple Authorized Service Provider and they can likely help. If you are not aware of any in your general area please click Apple Authorized Service Provider Locator (AASP) & Genius Appointment.
Finally, the computer is 5+ years old, if you are interested then another option is to replace the old base model iMac with a Mac mini and an external display. You will still need a Time Machine backup drive for the old computer but you can use that for a) migrating from the old computer to the new one and b) as a Time Machine drive for the new computer. A Mac mini will far outperform your current computer and for about $1500-$2000 you have future proofed your computer for another 7-10 years. If cash is short did you know Apple offers 12 month no interest financing?
Bottom line is you have some nice options available to you.
Best of luck!