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Click File in the menu bar, and select New Project... from the drop-down menu. In the setup window that appears, enter a name for your project. The aspect ratio refers to the width-to-height ratio of your movie, and will affect how it will appear on screens and monitors. Standard (4:3) is almost square, and is the ratio used in standard-definition TVs. A video in 4:3 will fill the screen of those TVs. On an HDTV or widescreen monitor, there will be black bars on either side of the picture so that it still looks correct. This is a good choice for standard TVs, and for iPad or iPhone playback. Widescreen (16:9) is much wider than it is tall, and is suitable for widescreen monitors and HDTV televisions. When viewed on a standard-definition monitor or TV, there will be black bars on the top and bottom so that it displays correctly. Chose one of the three options from the menu. Use the same frame rate that you used when you recorded your video you'll be using in your project. 30 fps - NTSC is the frame rate you will use if using a camera sold in North America, South America, Japan, and South Korea. 25 fps - PAL is the frame rate you will see in cameras sold in Europe, Hong Kong, and most other countries. 24 fps - Cinema is the setting you will use if you set your camera (either an NTSC or PAL camera) to record in Cinema mode. You can set a basic template for your movie using Themes. Click on each thumbnail, and in the preview area to the right, you'll see a sample video of how that theme looks in use. Select "Automatically add transitions and titles" if you want iMovie to insert standard cross-dissolve transitions with occasional theme-styled transitions between the clips in your project. If you don't choose this option, you can still add theme-styled transitions to your project manually. This will finalize your choices, and take you back to the main iMovie window's Project browser, where you can begin to build your project:

Summary:
Launch iMovie. Name your project. Assign an aspect ratio. Set your frame rate. Set your project theme. Click Create.