Hi @skhumbuzo90945
According to this [https://www.kingston.com/en/memory/search/model/77803/lenovo-essential-g580|link] that laptop comes standard with either 2GB or 4GB of ram and that the laptop supports up to 16GB of ram in 2 slots i.e. 2 x 8GB ram modules.
Search online for ''2 x 8GB DDR3 1600MT/s Non-ECC Unbuffered SODIMM ram'' to find suppliers that suit you best.
Here's the [https://download.lenovo.com/consumer/mobiles_pub/lenovo_g480_g485_g580_g585_g780_hmm_v1.0_english.pdf|hardware maintenance manual] for the laptop.
Go to p.45 (p.49 of 124 pdf) to view the procedure to remove /replace the DIMM (dual inline memory module i.e. ram)