Film advice

Hello Everybody,

I am new here and could really use some help deciding on a film. I am doing photographs for my friends engagement and was wondering on types of film and aperture settings. We will be most likely shooting outside and will probably be a sunny day. I am using an Pentax MZ-30 with a standard lens or maybe a 100mm(I think thats what it is) what type of film would you suggest? Standard 400 iso? or something lower/higher? And this will undoubtedly mark me as an newbie but what are the "rules" for using a higher or lower aperture setting than that of the film/the effects on the finished product.

Thanks a million and a half.

Josh.