You've screwed up at least the first couple of frames. Load it and advance, say, to frame 4, and start shooting there.
Okay--120 film is taped to a paper backing. You should load and unload it in subdued light, but total darkness is not necessary.
Different cameras load in different ways, but there are a few general options.
If the camera has an automatic frame counter (most modern cameras and SLRs) load the film on the feed side and attach the paper leader to the takeup spool (a used feed spool from a previous roll of film--you don't rewind medium format film), and advance the film until you see two large arrows oriented perpendicularly to the direction of film travel. Those arrows should point to two dots or arrows in the film chamber that indicate the start point. Close the back and advance to frame 1.
If the camera is an old camera with a red window, like a folder or older TLR, do the same thing, but there won't be a start point indicated inside the camera. When the paper backing is securely attached to the takeup spool, close the back, and advance the film until you see the number "1" in the red window. Note that this is strictly a manual operation, so it won't stop automatically, and if you miss it, you miss it. Some films (Kodak and most of the East European films) have clear markers indicating that the number is coming up. With some films, the numbers can be a bit hard to read, and they are easier to miss.