It depends. If it were a proper job I'd probably wear a suit. I've had lots of jobs (retail, care home, cafe, restaurant) and worn my every-day clothes to each of them. I haven't had an interview where I haven't got the job, but they weren't particularly important jobs so they probably would've hired anyone, anyway 
If I were looking to work like, in an office over summer, then I'd be smart (trousers, shirt) but wouldn't bother buying a suit.
If I were looking to start a career, then I'd buy a suit, definitely.

If I were looking to work like, in an office over summer, then I'd be smart (trousers, shirt) but wouldn't bother buying a suit.
If I were looking to start a career, then I'd buy a suit, definitely.