おしっこ means pee or urination. It is soft, casual, and often sounds childish.
It is common with children and pets, and adults may use it at home with close family. In normal adult conversation, though, it can sound too childish or too direct.
Use it as a noun, or with する:
おしっこをする = to pee
Similar words have different tones:
おしっこ: soft, homey, childish
小便（しょうべん）: rough or crude, often masculine
尿（にょう）: medical or clinical, meaning urine
Usage warning: Avoid おしっこ in polite, public, or adult conversation. Instead, say you are going to the トイレ, or use お手洗い（おてあらい） for restroom.