From what you say, I gather that you're visiting places without an appointment, just in the hope that they might have a vacancy.

To give a good first impression, make sure that your appearance is smart and tidy, adopt a good posture, be confident, look them in the eye when you speak to them, and generally try to look hard-working, honest and reliable (however you do that!).

First, make sure that you're speaking to the right person. "Hi, could I speak to the manager please" would be a good way to start. If the actual manager is not there then ask to speak to "the person in charge".

Once you're speaking to the manager you could say something like "Hi, I'm looking for work and I wondered if you had anything available?". If you're looking for a particular type of work then say "Hi, I'm looking for work as a waiter/dishwasher/whatever...."

As I'm sure you know, some places put notices in the window saying "waiters needed", or whetever it might be. In that case, you could say "Hi, I'm interested in the waiter's job/dishwashing job/whatever that you advertised outside."

If they say "yes" then great. If they initially say "no" then there's probably little hope and little point in carrying on the conversation for very long, but you could try something like "I work really hard, and I'm very reliable...", or, as a last resort, "I've got no money and I desperately need a job". Remember to be polite if they turn you away: "OK, well thanks anyway" would be appropriate.

If they want to contact you later with a decision and they ask for your phone number but you don't have one, you could say "Sorry, I don't have a phone at the moment. Can I come back later to get your decision?". Or, if you are able to phone them from a public phone, you could say "Can I call you later to get your decision".