In Italian there are several ways to express the concept of serving or helping oneself. The verb servire means "to serve":
Il cameriere serve il caffè ai clienti. (The waiter serves coffee to the customers.)
The phrase servirsi da, which literally means "to serve oneself" refers to the concept of helping oneself:
Grazie lo stesso; ci serviamo da soli. (Thank you anyway; we’ll serve (help) ourselves.)
The phrase servirsi di means "to use" or "to avail oneself of":
Per adesso mi servo di questi libri. (For now I use these books)
And finally, the phrase non servire a nulla means "to be of no use":
Queste cose non servono a nulla. (These things are useless.)