Just about everyone makes mistakes when they get into large format, so don't be discouraged. It will all feel worth it when you print your first good negatives.
First, read the page on film loading, which gives very clear step-by-step instructions at:
As everyone is saying, the whole process must be done in total darkness--no safelight. Take some of your black sheets of film and practice before trying with fresh film. Be sure the emulsion side of the film is facing out.
Next, be sure you've got the sequence correct when you take your pictures:
1. Set up and compose the image, focus, determine exposure and set the aperture and shutter speed. 2. Close the shutter. 3. Insert the film holder. 4. If the shutter needs to be cocked manually before firing, cock the shutter. If you have a self-cocking shutter, move on to the next step. 5. NOW remove the darkslide. Be sure it's the one between the film and the lens. 6. Fire the shutter. 7. Put the darkslide back in. 8. Remove the filmholder.
Now you're back in the darkroom. Again you need to remove the film from the holders and develop the film in total darkness (well, you could use a very dim blue-green safelight, but it is so dim, it's not really worth it). If you are using a daylight tank, you can turn on the lights when the film is loaded. If you are using trays, you need to do the whole process in the dark, without a safelight.
Be patient and check everything twice. You'll get the hang of it.