Step 1. Make iMovie slideshow from iPhoto library
iMovie works seamlessly with iPhoto. You are suggested to drag and drop photos from iPhoto to iMovie project. If no any library exists in iPhoto, create one and import photos to iPhoto first. To add videos, you could go to either "File/Import" or Event Library, and then drag and drop selected videos to the timeline.
After photos and videos are imported to iMovie slideshow project, you can preview the raw slideshow simply by pressing Space bar on your keyboard. And then make adjustments to clips and transitions. For example, mouse over one clip or "double arrow" button and click on the settings icon, then you could access clip adjustment, video adjustment, transition adjustment, etc.