Q: How can I backup my photos and delete it from my iPhone

What do you have available to back up to.

To increase space available, go Settings/iCloud/Photos and select Optimize iPhone/iPad Storage. That will reduce the space required by about 90% when fully effective.

external hard drive.

and how can i be sure that my photos are in the cloud before i delete from my phone

If you just want to copy them to your external hard drive instead of using iCloud, see this link:

Import photos and videos from your iPhone, iPad, or iPod touch to your computer - Apple Support

Then you can delete them from your iPhone:

Photos > Camera Roll

and select a picture and tap the trash can.