Pretty much the same kinds of things that make us laugh. Babies as young as 2 to 3 months will smile broadly if you look straight at them and smile at and talk to them. Then if you tickle them or jiggle their feet or hands, they will probably add sound to their reaction. This is the earliest kind of stimulation that will generally elicit a true laugh from a baby. It has been observed in children as young as 4 months but usually doesn't occur until later. Between 1 and 2 years, they will laugh in almost any kind of horse play, at something they have done (like throw a ball, dance around to music, etc.), and especially if they see or hear someone else laugh. By the time they are 3 years old, they will laugh at certain stories, pictures, TV programs intended to be funny, pictures of themselves, etc. By the time they are four, they will laugh at anything. Just observe circle time in any early childhood class if you want to see this.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.