Hello andreea175,
Thank you for posting in Apple Support Communities. We understand that you are wanting to download all of your photos to your computer and then upload them all to iCloud. We would love to provide you support on this.
This resource tells you how to import your photos from your iPhone to your Mac or PC: Transfer photos and videos from your iPhone, iPad, or iPod touch
This resource tells you how to set up iCloud photos on your devices and even has a link in the last section to show you how to set it up on a PC if needed: Set up and use iCloud Photos
This resource helps you if you run into issues with iCloud photos: Get help with iCloud Photos
Thank you for allowing us to support you.
Have an awesome and productive day!