So you've decided to use a film camera. What film fits your camera then?
Size? 35mm is the most common. It's also known as 135. For medium format (MF) cameras, there is 120 and 220. If your MF camera is old and has a window at the door, use 120 film with paper backing to prevent exposure from the back. If your camera is relatively new, you use either. 220 has the same width as 120 without the paper backing and doubly longer for more shots. Other film sizes for large format cameras are still available at large photo supply shops (maybe like B&H).
Color or B/W? If you like to shoot in color, buy color film. If you like the nostalgic effect of black and white, get black and white film. Is there any other issue left?
Slide or Negative? The most popular and available at most stores is negative film. It's also the easiest to use and easiest to find a developer for. It's called negative because after developing, the colors are somewhat reversed. When you print the pictures on paper, the colors come out right so don't worry about it.
Slide film or Chrome film is not very forgiving with exposure errors so this type of film is usually used only by those who know the basic principles of photography. When developed, the film contains the colors as you saw it. It can be mounted on a slide holder then viewed through a projector. This was the favorite film of college students presenting their thesis before MS Powerpoint became available.
Where to buy? Drugstores, corner stores, groceries, supermarkets and photo supply shops. It would be good to buy at a photo supply shop so you know where to get it developed too. If you bought it somewhere else, ask the clerk where's the nearest lab you can get it developed. They should have an idea.
Expiration date? Be sure to check it. It's written on the box somewhere. If you are not going to use the film yet, store it in your refrigerator. Expired film is usually cheaper but don't get one that's more than a year old.
Now you know!