Test Site/Sub Domain

Very new to wordpess and just doing some research that I can pass on to my IT guy. Basically I am looking for a place where I can fool around with our site, makes changes editing, e.c.t with out affecting our actual site. The people who host our site (Site5), instructed me that a sub domain would be best. After passing that on to my IT guy he set it up, but when you try going to it you are redirected to the main site… Site5 help said that it is cause by our sub domain and main site having the same data base. She then instructed me that she recommended going through a webapp to change this? Can anyone in basic English explain to me what my best option is and how to perform that??

https://kb.site5.com/domains/how-to-setup-a-subdomain/