To do this, you can use the INDEX keyword. 
N::NodeType {
  INDEX field1: String,
  field2: U32
}
This internally creates a table for the indexed field. To query a node by the indexed field, you can use object syntax: 
QUERY getByIndex(index_field: String) {
    node <- N<User>({field1: index_field})
    RETURN node
}
Note: You can currectly only use indexing with nodes. Indexing for edges and vectors is not supported yet.