When was cancer first ever treated and how did they know what to use?

Surgery: It's the obvious thing to try when you see an inexorably expanding lesion. The Romans were quite successful in treating early breast cancer surgically and got cures. Cautery was used in the middle ages and probably before for skin cancers.

The two lands: There are Egyptian papyri that deal with medical issues. Mostly they deal with trauma but bone cancer and female organ cancers were mentioned (or at least sounds like they were talking about it). Some were treated with surgery and others with potions and herbal concoctions (as well as spells) but most were addressed as "an illness I will not contend with" (essentially ancient hospice).