Originally posted by icy I only have a Pentax P30, so yes it's 35mm that's all I'm thinking of
I have a Pentax P30T, so I am exactly in your shoes. Be careful when using old or unusual film that doesn't have DX coding (the metallic strips on the cannister), because the camera will default to ISO/ASA 100 in that case. Now if it's 100 (or 125 is probably close enough) to start with, that's fine, but if it isn't then you will be consistently overexposing or underexposing (depending on what the film is) unless you take special care to adjust shutter speed or aperture to match.
If you're doing it for fun there's no harm in using something ordinary, even humble Kodacolor Gold (or its equivalent, depending on availability in your market). If you come across any of Lomography's colour negative films (the ordinary ones, not the ones with the weird and funky colour casts) and the price is right, my experience has been that they are just fine. Don't sneeze at expired film either, though you'll probably need to overexpose that a bit as it tends to "slow down" (I think the rule of thumb is about one stop per decade expired). I managed to pick up a whole lot of just-expired or just-about-to-expire film going out the door cheap last year, and I shot it without any serious problems (except the standard of my own photography!!).