Welcome to the Support Communities!
I'll try to provide a quick answer to your question, and direct you to some learning resources. You should be able to see your iPhoto library on the left side of your window in iMovie. If you don't see it, you may need to Unhide the Sidebar or from the menu bar choose Window > Show Libraries. You can drag your photos or video clips from the libraries in the Sidebar into your iMovie project.
Here are learning resources for iMovie:
I hope this information helps ....
Have a great day!