It’s been years since I used a Holga (120N), but they are really fun, and have a bit of an instinctive, “instant-art” type appeal to them.
The fun of the Holga is that it’s unpredicatble. Framing your photo is approximate. There are two apertures available: f/8 and f/11. So far as I can tell, that’s a lie. First, it’s only a one-stop difference, so it’s not going to make a huge difference in terms of either depth of field or exposure (unless you’re shooting slides). Second, the aperture selector doesn’t seem to actually change the aperture at all. Maybe I just had a broken Holga, but the exposures were exactly the same at f/8 and f/11 and when looking through the camera with no film, the size of aperture did not change. As far as I can tell the f/8 and f/11 markings are purely decorative.
You have one shutter speed (well, there is a "B"ulb mode, too) that is supposedly 1/100 or 1/125 sec, depending on whom you believe. Any technical specifications should be taken with a grain of salt and keeping in mind these are not engineered with great precision, so may vary from model to model.
The camera is also famous for its heavy vignetting and light leaks. This is part of the signature look of a Holga. The 120N comes with a 6x4.5 frame mask that eliminates some of the more egregious leaking and vignetting, but why would you want to do that? The fun and look of the Holga comes from those imperfections.
So, basically, you have a photographic instrument where you really only have one setting. Somewhere around 1/100-ish sec @ f/8-11ish. And with a viewfinder that only kind of sort of approximates what part of the picture might end up on film. The farther away you are from your subject the more accurate the viewfinder is (due to parallax), but you can’t count on very finely controlling what is on the edges of your frame, except with a lot of experience. Focus is similarly a shot in the dark. There’s a few markings with figures signifying “single person portrait, small group, large group, and landscapes” which are supposed to correspond with the distance you probably are at to take those pictures, but it’s all kind of approximate and takes guesswork and trial and error to get down.
Now, if you’re basically stuck with one or possible two exposures (if your version of the Holga actually does have two aperture settings that work, and disregarding the "B"ulb setting for the moment), this means you have to be very careful about what lighting conditions you shoot in and what film you use. If you’re shooting on a sunny day, go out with 100 speed film, and you will be fine. You may be overexposing by 1 stop or so, but that will print okay. If it’s overcast, you’d better be shooting 400 speed film. Maybe even 800, depending on how heavy the cloud cover is. Basically, the only way you can control exposure is with film speed, so you do have to have some sort of sense of how film speed is related to exposure.
So, to keep it simple, take 100 speed film out and shoot a roll with it on a sunny day. If it’s cloudy, try 400 or 800. See how it goes. I would stay away from indoors photos unless you are using a flash, have some idea of how to use Bulb modes and keep the camera steady. Just point it at something interesting, approximate the focus distance, trip the shutter and pray. Most of the fun, for me, is not knowing exactly what you’re going to get.