Pitch your idea to friends and family you trust who aren't in the entertainment business(you don not want your idea stolen). Get their feedback. Listen to their comments about things that would be interesting to see and ways to make it better. Write out your idea on paper.
Purchase a screenplay software. Write out your script as you see it happening with your host's speeches, commercial breaks, challenges and more. Since it is a reality show you will have lots of open space but at least script out the basic bones of the show's structure, look and feel.
Write out several more potential scenarios, challenges and twists on a separate document so you can show where the show is headed and what you have planned or what's in store for the rest of the series should it continue. Find or hire a crew to shoot the pilot episode. Once your crew has been found (friends and family work fine or you can hire professionals), it's time to find the candidates for your show.
Hold an open casting call. Audition and interview everyone who comes in. Videotape the auditions so you can both see how they look on camera and have something to reference and look back on when making your decision on who to choose. Review all the potential candidates and make your selections. Make sure you have a good mixture of people from different backgrounds, social circles and walks of life. This is what will create conflict and drama and keep things interesting.
Shoot your pilot episode. This may mean setting up a studio or following people around, depending on what the premise of your reality show is. Edit the footage on digital computer editing software and you have a reality show. Post it on YouTube, use it to pitch to studios and investors or just keep in your own personal video library.