To know when use “les” or “des”, you simply have to follow these grammar rules (same rules in english):

1. definite article/article défini : the/le, la, les : these articles are used when we know the person, animal or object we are talking about.

2. indefinite article/article indéfini : a, an, some, any/un, une, des : these articles are used when we know nothing about the person, animal or object we are talking about.

Also both phrases are corrects :

• “Il y a des gens”/there is people : it means you don’t know anything at all about these people.
• “Il y a les filles”/there is the girls : it means you know these girls.
  For example : entering in a bar : “Cool, il y a les filles !” : you’re happy, your gang composed by girl is her.

Hoping to help you