Years ago (I think in my late teens or early twenties), a friend invited me to take photographs for a park district's promotional material.
It was pretty casual. I don't think we even really set a price--which was good, because it kind of fell apart since neither side knew what we were doing. I knew I could legally photograph anyone in a public setting, but didn't have knowledge or experience of what kind of releases or permission you'd need for advertising.
On their end, they didn't give me any guidelines past a "people enjoying parks" kind of idea. No number of images, what they'd be used for, etc. I think I took some pictures, but suffered from a lack of direction. But I am happy that I was honest and let them know from the beginning that I wasn't a pro. Looking back, I should have asked for advice from a teacher. I think at the time, I would have been hesitant to bother him, but now I realize he would have been thrilled to give me advice.
My recommendations would be to do research and ask the client lots of questions, especially if they don't seem to know what they want, and to ask others for advice or help.