You say "There you go" when you're giving or handing a person something:

A: Can you pass me a napkin?

B: There you go.

More generally, you use it when you're doing something for another person. For example, say "There you go" when:

• you stand up to let someone sit down
• you've finished tying someone's necktie for them
• you clear a space on the kitchen counter for someone to set a hot dish on

You can also say "Here you go" in the same way. There are no absolute rules for when to use one or the other, but I would use "Here you go" if I brought something to them from far away, and "there you go" if I'm sitting in one place and handing them the object.