Besides can either be a preposition (1) or an adverb (2)
1. As a preposition besides means "in addition to". It precedes a noun/pronoun/gerund:
- Besides working as a librarian she also looks after the garden.
2. As an adverb besides (adverb) means "in addition". It usually precedes its clause, but can follow it:
I can't go to the cinema; I'm busy . Besides, I don't have any money.
Note that moreover could replace besides here in more formal English.
Also, anyway or in any case could be used here in more informal English:
- Anyway, I don't have any money .
