Hey, /n/. I need some help.

I've never taken a public bus before. Now I've been starting to take one this past week, but I can't figure out how I'm supposed to stop at the stop I'm supposed to get off at. I asked the bus driver the first time I got on, then he dropped me off at the stop I needed. Then I got again, but this time he doesn't stop at all the stops, and he skipped mine for about 5 days. I got confused because I thought bus drivers were supposed to stop at each bus stop? Well, anyway, I'm pretty bewildered with all this of this. My friend told me there's some kind of string that you pull if you want to get off at a stop. Is this true? And if it is, where can I locate this string? I don't think I've ever seen it before on the bus, but maybe I just wasn't paying attention.

Any help would be appreciated.