"To be more specific" about something is to figure out the details related to the issue. So the writer wanted to be 'somebody', but they never considered the details of what kind of person they wanted to be. The writer is realising they should have done it before.

Normally it means you have asked someone to do something, but they haven't done what you wanted. For example, if I asked a flatmate to buy some fruit and they only bought two apples when I wanted apples, oranges and grapes I could say I should have been more specific.

BE MORE SPECIFIC is literal. BE SOMEBODY is the phrase that's an expression. It means to be a person who is highly regarded in some field or activity. So, the writer is kind of making a joke. She is now somebody (in the literal sense--everyone is), but she is not the sort of person that she wants to be