The bike you should get depends a lot on your use case. A used one is a decent choice if you're doing short commuting and random city errands. If you want to do long or fast rides, invest more (though beware there's no real ceiling on bike cost and accessories have strong allure).

Whatever bike you get, make sure it's in decent shape and is sized correctly for you. Also put a bit of effort into maintenance (lube the chain and inflate the tires and you're fine for casual riding). AND GET SAFETY LIGHTS.