"Ta-da!" is the sound that a magician traditionally makes when he does a magic trick, like pulling a rabbit out of his hat.
In modern days, people say this when they reveal a surprise, especially a happy surprise. For example, if you get dressed up really nicely, you can say "Ta-da!" when you show your clothes to your boyfriend or girlfriend. You can also say "Ta-da!" when you unexpectedly give a person something that they wanted:
You know how you've been talking about wanting a new teapot? Well, ta-da!