If my photos are in the iCloud. When I delete photos from my phone will they also be deleted from the iCloud?

iCloud photo is a synchronization service, so yes, deleting from your phone will delete in iCloud.

To preserve those pictures, you will need to transfer them to some archival storage first, then delete them. See - Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

The simplest way to preserve photo space, without compromising on photo/video quality, is to navigate Settings:Photos:select “Optimize iPhone Storage.” This just keeps a tiny thumbnail of the photo on your device, while keeping the full-resolution originals safely stored in iCloud. The lower-resolution local copy will automatically revert to the full-quality version if you need to adjust it, before syncing the full-quality version to the cloud and then re-replacing the local copy with a tiny placeholder. It’s so seemless, you’ll never have to think about it.