When buying a flash look at the manual or ask what the guide number is when teamed with different kinds of film. As a example when you use 400-speed film with a vivitar 283 flash, you will get a guide number of 240. I have used this flash for a long time and is dependable. It is also inexpensive. After you have a flash, READ the instructions and take several practice rolls of film to get used to it, go to a drill at night and try all of the settings to see which works right.
Flashes run on batteries.
Normal batteries are cheap, but do not last long. You should consider buying a battery pack to power your flash. This was one of the first things that I bought. You cannot take pictures at night if your flash runs out of batteries. Even if you have a battery pack you should still carry back up batteries.