I always use ISO 400 film, either BW or color negative. ISO 400 film yields negatives that are a bit overexposed in very bright sunlight, and a bit underexposed in overcast conditions -- but in both cases entirely useable.
ISO 100 film routinely yields underexposed negs, which are harder to work with -- basically, theres NO detail to be salvaged from underexposed areas that are blank on the film, whereas you can usually bring in detail from overexposed areas that are dense on the film.
For what its worth, I shoot Kodak Portra 400UC (or sometimes its NC VC variants, which work equally well). I used to shoot T-Max 400 for BW, but now I shoot only color film and then downshift to BW as desired when I scan the negs.
What I have also found useful sometimes is attaching a small strobe when photographing outdoors in dull light. (The one high-tech feature of the Holga that actually works is its hot shoe atop the camera.) The basic light from the scene is adequate to record the overall view, and the flash provides fill-in light to brighten whatever subject matter is relatively close to the camera. The effect is entirely natural-looking once youve experimented with it a bit.
If you want to work indoors, youll almost certainly need a flash -- and probably one thats more sophisticated flexible than the one thats built in to the Flash [model] Holga. Alternatively, you could experiment with high-speed films (ISO1200-3200), but personally Ive found that the range of indoor lighting is so varied that simply using high-speed film rarely ensures a usable exposure. In those circumstances you need to either control the amount of light in the scene (by adding some with the flash) or control the shutter speed or aperture on the camera (which is not an option with the Holga).
Hope this helps.
With Best Wishes,