Inspiron 3515 can use up to 32 GB RAM (two x 16 GB) DDR4 at 2400 MHz. Here is RAM from Crucial that is guaranteed compatible with your system.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)

FYI: You didn't say whether it has a hard drive or an SSD. If it's a hard drive, replacing it with an SSD will make it much faster and more responsive, a noticeable difference and the most bang-for-your-buck that you can get for any upgrade. If you already have 8 GB RAM, replacing a hard drive with an SSD will make a more significant performance improvement than adding more RAM.