What format should I used when creating

Hello All. When creating a project, i am importing videos in various formats ( .mp4, .mov, .avi. etc.) What format can I use, or what settings within FCP can I adjust to make the project play smoother when working and editing it? I always have to wait for the videos to do something before I can see them. Also what format should I save my final videos ( export)? They are usually going to video sites like Youtube.
Thanks

Assuming you start with the media files already on a hard disk, import using "leave files in place". See FCPX Preferences>Import. That makes the import much faster since it avoids copying the media inside the library.

If it will not play smoothly, try setting the viewer to "better performance". If that is not sufficient you'll need to create proxies or optimized media.

To export for Youtube, I suggest File>Share>Master File>Settings, Format: Computer, Video Codec: Faster Encode, Resolution: 1920x1080. This lets you examine the output file and verify the quality before uploading to Youtube.